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Indian indigo dye lump.jpg
A piece of indigo plant dye from India,
about 6 cm (2.5 in) square
Wavelength 450–420 [1] (disputed) nm
Gtk-dialog-info.svg    Color coordinates
Hex triplet #3F00FF
sRGB B  (r,  g,  b)(63, 0, 255)
CMYK H  (c, m, y, k)(75, 100, 0, 0)
HSV     (h, s, v)(255°, 100%, 100%)
Source HTML/CSS [2]
B: Normalized to [0–255] (byte)
H: Normalized to [0–100] (hundred)

Indigo is a deep and rich color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine. It is traditionally regarded as a color in the visible spectrum, as well as one of the seven colors of the rainbow: the color between violet and blue; however, sources differ as to its actual position in the electromagnetic spectrum.

RGB color space any additive color space based on an RGB color model

An RGB color space is any additive color space based on the RGB color model. A particular RGB color space is defined by the three chromaticities of the red, green, and blue additive primaries, and can produce any chromaticity that is the triangle defined by those primary colors. The complete specification of an RGB color space also requires a white point chromaticity and a gamma correction curve. As of 2007, sRGB is by far the most commonly used RGB color space.

Ultramarine A deep blue color pigment which was originally made with ground lapis lazuli

Ultramarine is a deep blue color pigment which was originally made by grinding lapis lazuli into a powder. The name comes from the Latin ultramarinus, literally "beyond the sea", because the pigment was imported into Europe from mines in Afghanistan by Italian traders during the 14th and 15th centuries.

Visible spectrum Portion of the electromagnetic spectrum that is visible to the human eye

The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 740 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 430–770 THz.


The color indigo is named after the indigo dye derived from the plant Indigofera tinctoria and related species.

Indigo dye chemical compound; food additive and dye

Indigo dye is an organic compound with a distinctive blue color. Historically, indigo was a natural dye extracted from the leaves of certain plants, and this process was important economically because blue dyes were once rare. A large percentage of indigo dye produced today, several thousand tonnes each year, is synthetic. It is the blue often associated with denim cloth and blue jeans.

<i>Indigofera tinctoria</i> species of plant

Indigofera tinctoria, also called true indigo, is a species of plant from the bean family that was one of the original sources of indigo dye. It has been naturalized to tropical and temperate Asia, as well as parts of Africa, but its native habitat is unknown since it has been in cultivation worldwide for many centuries. Today most dye is synthetic, but natural dye from I. tinctoria is still available, marketed as natural coloring where it is known as tarum in Indonesia and nila in Malaysia. In Iran and areas of the former Soviet Union it is known as basma. The plant is also widely grown as a soil-improving groundcover.

The first known recorded use of indigo as a color name in English was in 1289. [3]

English language West Germanic language

English is a West Germanic language that was first spoken in early medieval England and eventually became a global lingua franca. It is named after the Angles, one of the Germanic tribes that migrated to the area of Great Britain that later took their name, as England. Both names derive from Anglia, a peninsula in the Baltic Sea. The language is closely related to Frisian and Low Saxon, and its vocabulary has been significantly influenced by other Germanic languages, particularly Norse, and to a greater extent by Latin and French.


Extract of natural indigo applied to paper Indigo plant extract sample.jpg
Extract of natural indigo applied to paper

Species of Indigofera were cultivated in East Asia, Egypt, India, and Peru in antiquity. The earliest direct evidence for the use of indigo dates to around 4000 BC and comes from Huaca Prieta, in contemporary Peru. [4] Pliny the Elder mentions India as the source of the dye after which it was named. [5] It was imported from there in small quantities via the Silk Road. [6]

Egypt Country spanning North Africa and Southwest Asia

Egypt, officially the Arab Republic of Egypt, is a country in the northeast corner of Africa, whose territory in the Sinai Peninsula extends beyond the continental boundary with Asia, as traditionally defined. Egypt is bordered by the Gaza Strip and Israel to the northeast, the Gulf of Aqaba and the Red Sea to the east, Sudan to the south, Libya to the west, and the Mediterranean Sea to the north. Across the Gulf of Aqaba lies Jordan, across the Red Sea lies Saudi Arabia, and across the Mediterranean lie Greece, Turkey and Cyprus, although none share a land border with Egypt.

India Country in South Asia

India, officially the Republic of India, is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the southwest, and the Bay of Bengal on the southeast, it shares land borders with Pakistan to the west; China, Nepal, and Bhutan to the north; and Bangladesh and Myanmar to the east. In the Indian Ocean, India is in the vicinity of Sri Lanka and the Maldives; its Andaman and Nicobar Islands share a maritime border with Thailand and Indonesia.

Peru Republic in South America

Peru, officially the Republic of Peru, is a country in western South America. It is bordered in the north by Ecuador and Colombia, in the east by Brazil, in the southeast by Bolivia, in the south by Chile, and in the west by the Pacific Ocean. Peru is a megadiverse country with habitats ranging from the arid plains of the Pacific coastal region in the west to the peaks of the Andes mountains vertically extending from the north to the southeast of the country to the tropical Amazon Basin rainforest in the east with the Amazon river.

The Ancient Greek term for the dye was Ἰνδικὸν φάρμακον ("Indian dye"), which, adopted to Latin as indicum and via Portuguese gave rise to the modern word indigo . [7]

Ancient Greek Version of the Greek language used from roughly the 9th century BC to the 6th century AD

The ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often roughly divided into the Archaic period, Classical period, and Hellenistic period. It is antedated in the second millennium BC by Mycenaean Greek and succeeded by Medieval Greek.

Latin Indo-European language of the Italic family

Latin is a classical language belonging to the Italic branch of the Indo-European languages. The Latin alphabet is derived from the Etruscan and Greek alphabets and ultimately from the Phoenician alphabet.

Portuguese language Romance language that originated in Portugal

Portuguese is a Western Romance language originating in the Iberian Peninsula. It is the sole official language of Portugal, Brazil, Cape Verde, Guinea-Bissau, Mozambique, Angola and São Tomé and Príncipe. It also has co-official language status in East Timor, Equatorial Guinea and Macau in China. As the result of expansion during colonial times, a cultural presence of Portuguese and Portuguese creole speakers are also found in Goa, Daman and Diu in India; in Batticaloa on the east coast of Sri Lanka; in the Indonesian island of Flores; in the Malacca state of Malaysia; and the ABC islands in the Caribbean where Papiamento is spoken, while Cape Verdean Creole is the most widely spoken Portuguese-based Creole. A Portuguese-speaking person or nation is referred to as "Lusophone" (Lusófono).

Spanish explorers discovered an American species of indigo and began to cultivate the product in Guatemala. The English and French subsequently began to encourage indigo cultivation in their colonies in the West Indies. [8]

Guatemala Republic in Central America

Guatemala, officially the Republic of Guatemala, is a country in Central America bordered by Mexico to the north and west, Belize and the Caribbean to the northeast, Honduras to the east, El Salvador to the southeast and the Pacific Ocean to the south. With an estimated population of around 16.6 million, it is the most populated country in Central America. Guatemala is a representative democracy; its capital and largest city is Nueva Guatemala de la Asunción, also known as Guatemala City.

West Indies Island region of the North Atlantic Ocean and the Caribbean

The West Indies is a region of the North Atlantic Ocean and the Caribbean that includes the island countries and surrounding waters of three major archipelagos: the Greater Antilles, the Lesser Antilles, and the Lucayan Archipelago.

Blue dye can be made from two different types of plants: the indigo plant, which produces the best results, and from the woad plant Isatis tinctoria , also known as pastel. [9] For a long time woad was the main source of blue dye in Europe. Woad was replaced by true indigo as trade routes opened up, and both plant sources have now been largely replaced by synthetic dyes.

Classification as a spectral color

Indigo is one of the colors on Newton's color wheel Newton's colour circle.png
Indigo is one of the colors on Newton's color wheel

The Early Modern English word indigo referred to the dye, not to the color (hue) itself, and indigo is not traditionally part of the basic color-naming system. [10] Modern sources place indigo in the electromagnetic spectrum between 420 and 450 nanometers, [1] [11] [12] which lies on the short-wave side of color wheel (RGB) blue, towards (spectral) violet.

However, the correspondence of this definition with colors of actual indigo dyes is disputed. Optical scientists Hardy and Perrin list indigo as between 445 [13] and 464 nm wavelength, [14] which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards azure.

Isaac Newton introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of prisms at a fair near Cambridge, the East India Company had begun importing indigo dye into England, [15] supplanting the homegrown woad as source of blue dye. In a pivotal experiment in the history of optics, the young Newton shone a narrow beam of sunlight through a prism to produce a rainbow-like band of colors on the wall. In describing this optical spectrum, Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations." [16] He linked the seven prismatic colors to the seven notes of a western major scale, [17] as shown in his color wheel, with orange and indigo as the semitones. Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall:

Newton's observation of prismatic colors. Comparing this to a color image of the visible light spectrum will show that indigo corresponds to blue, while blue corresponds to cyan. Newton prismatic colours.JPG
Newton's observation of prismatic colors. Comparing this to a color image of the visible light spectrum will show that indigo corresponds to blue, while blue corresponds to cyan.

I desired a friend to draw with a pencil lines cross the image, or pillar of colours, where every one of the seven aforenamed colours was most full and brisk, and also where he judged the truest confines of them to be, whilst I held the paper so, that the said image might fall within a certain compass marked on it. And this I did, partly because my own eyes are not very critical in distinguishing colours, partly because another, to whom I had not communicated my thoughts about this matter, could have nothing but his eyes to determine his fancy in making those marks. [18]

Traditional seven colors of the rainbow Rainbow-diagram-ROYGBIV.svg
Traditional seven colors of the rainbow

Indigo is therefore counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonic Roy G. Biv . James Clerk Maxwell and Hermann von Helmholtz accepted indigo as an appropriate name for the color flanking violet in the spectrum. [19]

Later scientists conclude that Newton named the colors differently from current usage. [20] [21] According to Gary Waldman, "A careful reading of Newton's work indicates that the color he called indigo, we would normally call blue; his blue is then what we would name blue-green, cyan or light blue." [22] If this is true, Newton's seven spectral colors would have been:

Red:    Orange:    Yellow:    Green:    Blue:     Indigo:    Violet:    

The human eye does not readily differentiate hues in the wavelengths between what we today call blue and violet. If this is where Newton meant indigo to lie, most individuals would have difficulty distinguishing indigo from its neighbors. According to Isaac Asimov, "It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes it seems merely deep blue." [23]

Modern color scientists typically divide the spectrum between violet and blue at about 450 nm, with no indigo. [24] [25]

Distinction among the four major tones of indigo

Like many other colors (orange, rose, and violet are the best-known), indigo gets its name from an object in the natural world—the plant named indigo once used for dyeing cloth (see also Indigo dye).

The color electric indigo is a bright and saturated color between the traditional indigo and violet. This is the brightest color indigo that can be approximated on a computer screen; it is a color located between the (primary) blue and the color violet of the RGB color wheel.

The web color blue violet or deep indigo is a tone of indigo brighter than pigment indigo, but not as bright as electric indigo.

The color pigment indigo is equivalent to the web color indigo and approximates the color indigo that is usually reproduced in pigments and colored pencils.

The color of indigo dye is a different color from either spectrum indigo or pigment indigo. This is the actual color of the dye. A vat full of this dye is a darker color, approximating the web color midnight blue.

Below are displayed these four major tones of indigo.

Electric indigo

Electric Indigo
Gtk-dialog-info.svg    Color coordinates
Hex triplet #6F00FF
sRGB B  (r,  g,  b)(111, 0, 255)
CMYK H  (c, m, y, k)(57, 100, 0, 0)
HSV     (h, s, v)(266°, 100%, 100 [26] %)
B: Normalized to [0–255] (byte)
H: Normalized to [0–100] (hundred)

"Electric indigo" is brighter than the pigment indigo reproduced below. When plotted on the CIE chromaticity diagram, this color is at 435 nanometers, in the middle of the portion of the spectrum traditionally considered indigo, i.e., between 450 and 420 nanometers. This color is only an approximation of spectral indigo, since actual spectral colors are outside the gamut of the sRGB color system.

Deep indigo (web color blue-violet)

Gtk-dialog-info.svg    Color coordinates
Hex triplet #8A2BE2
sRGB B  (r,  g,  b)(138, 43, 226)
CMYK H  (c, m, y, k)(63, 81, 0, 0)
HSV     (h, s, v)(271°, 81%, 89%)
Source X11
B: Normalized to [0–255] (byte)
H: Normalized to [0–100] (hundred)

At right is displayed the web color "blue-violet", a color intermediate in brightness between electric indigo and pigment indigo. It is also known as "deep indigo".

Web color indigo

Web color Indigo
Gtk-dialog-info.svg    Color coordinates
Hex triplet #4B0082
sRGB B  (r,  g,  b)(75, 0, 130)
CMYK H  (c, m, y, k)(42, 100, 0, 49)
HSV     (h, s, v)(247.6°, 100%, 51%)
B: Normalized to [0–255] (byte)
H: Normalized to [0–100] (hundred)

The color box at right displays the web color indigo, the color indigo as it would be reproduced by artists' paints as opposed to the brighter indigo above (electric indigo) that is possible to reproduce on a computer screen. Its hue is closer to violet than to indigo dye for which the color is named. Pigment indigo can be obtained by mixing 55% pigment cyan with about 45% pigment magenta.

Compare the subtractive colors to the additive colors in the two primary color charts in the article on primary colors to see the distinction between electric colors as reproducible from light on a computer screen (additive colors) and the pigment colors reproducible with pigments (subtractive colors); the additive colors are significantly brighter because they are produced from light instead of pigment.

Web color indigo represents the way the color indigo was always reproduced in pigments, paints, or colored pencils in the 1950s. By the 1970s, because of the advent of psychedelic art, artists became used to brighter pigments, and pigments called "bright indigo" or "bright blue-violet" that are the pigment equivalent of the electric indigo reproduced in the section above became available in artists' pigments and colored pencils.

Tropical indigo

Tropical Indigo
Gtk-dialog-info.svg    Color coordinates
Hex triplet #9683EC
sRGB B  (r,  g,  b)(150, 131, 236)
CMYK H  (c, m, y, k)(48, 51, 0, 0)
HSV     (h, s, v)(251°, 44%, 93%)
SourceGallego and Sanz [27]
B: Normalized to [0–255] (byte)
H: Normalized to [0–100] (hundred)

'Tropical Indigo' is the color that is called añil in the Guía de coloraciones (Guide to colorations) by Rosa Gallego and Juan Carlos Sanz, a color dictionary published in 2005 that is widely popular in the Hispanophone realm.

Indigo dye

Indigo Dye
Gtk-dialog-info.svg    Color coordinates
Hex triplet #00416A
sRGB B  (r,  g,  b)(0, 65, 106)
CMYK H  (c, m, y, k)(100, 39, 0, 58)
HSV     (h, s, v)(203°, 100%, 42%)
B: Normalized to [0–255] (byte)
H: Normalized to [0–100] (hundred)

Indigo dye is a greenish dark blue color.

Imperial blue

Imperial Blue
Gtk-dialog-info.svg    Color coordinates
Hex triplet #002395
sRGB B  (r,  g,  b)(0, 35, 149)
HSV     (h, s, v)(226°, 100%, 58%)
B: Normalized to [0–255] (byte)

In nature

Male indigobirds are a very dark, metallic blue.
The indigo bunting, native to North America, is mostly bright cerulean blue with an indigo head.
The related blue grosbeak is, ironically, more indigo than the indigo bunting.
Lactarius indigo is one of the very few species of mushrooms colored in tones of blue.
The eastern indigo snake, Drymarchon couperi, of the southeastern United States, is a dark blue/black.

In culture


Marina Warner's novel Indigo (1992) is a retelling of Shakespeare's The Tempest and features the production of indigo dye by Sycorax.


Computer graphics


Indigo is created in potholes carved in pumice "tufgrond" in Karoland, Sumatra COLLECTIE TROPENMUSEUM Indigo wordt aangemaakt in kuilen in uitgehakte puimsteentufgrond Karolanden TMnr 10014190.jpg
Indigo is created in potholes carved in pumice "tufgrond" in Karoland, Sumatra



The French Army adopted dark blue indigo at the time of the French Revolution, as a replacement for the white uniforms previously worn by the Royal infantry regiments. In 1806, Napoleon decided to restore the white coats because of shortages of indigo dye imposed by the British continental blockade. However, the greater practicability of the blue color led to its retention, and indigo remained the dominant color of French military coats until 1914.


The spiritualist applications use electric indigo, because the color is positioned between blue and violet on the spectrum. [34]

See also

Related Research Articles

Color Characteristic of human visual perception

Color, or colour, is the characteristic of human visual perception described through color categories, with names such as red, orange, yellow, green, blue, or purple. This perception of color derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum. Color categories and physical specifications of color are associated with objects through the wavelength of the light that is reflected from them. This reflection is governed by the object's physical properties such as light absorption, emission spectra, etc.

Yellow color

Yellow is the color between orange and green on the spectrum of visible light. It is evoked by light with a dominant wavelength of roughly 570–590 nm. It is a primary color in subtractive color systems, used in painting or color printing. In the RGB color model, used to create colors on television and computer screens, yellow is a secondary color made by combining red and green at equal intensity. Carotenoids give the characteristic yellow color to autumn leaves, corn, canaries, daffodils, and lemons, as well as egg yolks, buttercups, and bananas. They absorb light energy and protect plants from photodamage. Sunlight has a slight yellowish hue when sun is near a horizon, due to atmosphere scattering shorter wavelengths.

Violet (color) color

Violet is the color at the end of the visible spectrum of light between blue and the invisible ultraviolet. Violet color has a dominant wavelength of approximately 380–450 nanometers. Light with a shorter wavelength than violet but longer than X-rays and gamma rays is called ultraviolet. In the color wheel historically used by painters, it is located between blue and purple. On the screens of computer monitors and television sets, a color which looks similar to violet is made, with the RGB color model, by mixing red and blue light, with the blue twice as bright as the red. This is not true violet, for it does not match the color of a single wavelength shorter than that of blue light.

Purple Range of colors with the hues between blue and red

Purple is a color intermediate between blue and red. It is similar to violet, but unlike violet, which is a spectral color with its own wavelength on the visible spectrum of light, purple is a secondary color made by combining red and blue. The complementary color of purple in the RYB color model is yellow.

Magenta color visible between red and purple; subtractive (CMY) primary color

Magenta is a color that is variously defined as purplish-red, reddish-purple or mauvish-crimson. On color wheels of the RGB (additive) and CMY (subtractive) color models, it is located midway between red and blue. It is one of the four colors of ink used in color printing by an inkjet printer, along with yellow, black, and cyan, to make all the other colors. The tone of magenta used in printing is called "printer's magenta".

<i>Isatis tinctoria</i> species of plant

Isatis tinctoria, also called woad, dyer's woad, or glastum, is a flowering plant in the family Brassicaceae. It is occasionally known as Asp of Jerusalem. Woad is also the name of a blue dye produced from the leaves of the plant. Woad is native to the steppe and desert zones of the Caucasus, Central Asia to Eastern Siberia and Western Asia but is now also found in South-Eastern and Central Europe and western North America.

A lake pigment is a pigment made by precipitating a dye with an inert binder, or "mordant", usually a metallic salt. Unlike vermilion, ultramarine, and other pigments made from ground minerals, lake pigments are organic. Manufacturers and suppliers to artists and industry frequently omit the lake designation in the name. Many lake pigments are fugitive because the dyes involved are not lightfast. Red lakes were particularly important in Renaissance and Baroque paintings; they were often used as translucent glazes to portray the colors of rich fabrics and draperies.

In the visual arts, color theory or colour theory is a body of practical guidance to color mixing and the visual effects of a specific color combination. There are also definitions of colors based on the color wheel: primary color, secondary color, and tertiary color. Although color theory principles first appeared in the writings of Leone Battista Alberti and the notebooks of Leonardo da Vinci, a tradition of "colory theory" began in the 18th century, initially within a partisan controversy over Isaac Newton's theory of color and the nature of primary colors. From there it developed as an independent artistic tradition with only superficial reference to colorimetry and vision science.

Navy blue very dark shade of the color blue which almost appears as black

Navy blue is a very dark shade of the color blue.

Color wheel abstract illustrative organization of color hues

A colour wheel or colour circle is an abstract illustrative organization of color hues around a circle, which shows the relationships between primary colors, secondary colors, tertiary colors etc.

Spectral color color evoked by a single wavelength of light in the visible spectrum

A spectral color is a color that is evoked in a normal human by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths, also known as monochromatic light. Every wavelength of visible light is perceived as a spectral color, in a continuous spectrum; the colors of sufficiently close wavelengths are indistinguishable for the human eye.

Red-violet color

Red-violet is a rich color of high medium saturation about 3/4 of the way between red and magenta, closer to magenta than to red. It is classified in color theory as one of the purple colors—a non-spectral color between red and violet that is a deep version of a color on the line of purples on the CIE chromaticity diagram. Both its saturation and brightness falling short of 100%, red-violet is not a pure chroma. There is a color of similar hue that, however, comes close to being a pure chroma: process magenta. The pure chroma color composed of equal parts of magenta and red is called rose.

A tertiary color or intermediate color is a color made by mixing full saturation of one primary color with half saturation of another primary color and none of a third primary color, in a color space such as RGB, CMYK or RYB (traditional).

Shades of red Colors that are variations of red

Varieties of the color red may differ in hue, chroma or lightness, or in two or three of these qualities. Variations in value are also called tints and shades, a tint being a red or other hue mixed with white, a shade being mixed with black. A large selection of these various colors is shown below.

Line of purples

In color theory, the line of purples or purple boundary is the locus on the edge of the chromaticity diagram formed between extreme spectral red and violet. Except for these endpoints of the line, colors on the line are non-spectral. Rather, every color on the line is a unique mixture in a ratio of fully saturated red and fully saturated violet, the two spectral color endpoints of visibility on the spectrum of pure hues. Colors on the line and spectral colors are the only ones that are fully saturated in the sense that, for any point on the line, no other possible color being a mixture of red and violet is more saturated than it.

Shades of blue Variety of the color blue

Varieties of the color blue may differ in hue, chroma, or lightness, or in two or three of these qualities. Variations in value are also called tints and shades, a tint being a blue or other hue mixed with white, a shade being mixed with black. A large selection of these various colors is shown below.

Natural dye dye extracted from plant or animal sources

Natural dyes are dyes or colorants derived from plants, invertebrates, or minerals. The majority of natural dyes are vegetable dyes from plant sources—roots, berries, bark, leaves, and wood—and other biological sources such as fungi and lichens.

Shades of violet Varieties of the color violet

There are numerous variations of the color violet, a sampling of which are shown below.

Shades of purple Variations of the color purple

There are numerous variations of the color purple, a sampling of which are shown below.


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  28. "It's New and It's Blue" (Indigo advertisement), The Globe and Mail , Toronto, 1 October 1999, p. A3
  29. "Indigo Bookstore had a 'Think Blue' campaign back in 1999" according to: "Think Blue 2008: a Before and After Tale of Silly Turf Battles and Redemptive Communication" . Retrieved 4 February 2013.[ better source needed ]
  30. Kitchin, Thomas (1778). The Present State of the West-Indies: Containing an Accurate Description of What Parts Are Possessed by the Several Powers in Europe. London: R. Baldwin. p. 30.
  31. Gallego, Rosa; Sanz, Juan Carlos (2001). Diccionario Akal del color. Akal. ISBN   978-84-460-1083-8.
  32. Article „añil“ in: Enciclopedia de México, vol 1, Mexiko-City: Secretaría de Educacion Pública, 1987
  33. Wired Volume 19 No. 10 October 2011 Page 50
  34. Tansley, David W. Subtle Body: Essence and Shadow 1984 (Art and Cosmos Series--Jill Purce, editor)
  35. Stevens, Samantha. The Seven Rays: a Universal Guide to the Archangels. City: Insomniac Press, 2004. ISBN   1-894663-49-7 pg. 24
  36. Graham, Lanier F. (editor) The Rainbow Book Berkeley, California:1976 Shambala Publishing and The Fine Arts Museums of San Francisco (Handbook for the Summer 1976 exhibition The Rainbow Art Show which took place primarily at the De Young Museum, but also at other museums) Indigo Pages 152–153 The color indigo is stated to represent intuition.
  37. Bailey, Alice A. (1995). The Seven Rays of Life. New York: Lucis Publishing Company. ISBN   978-0-85330-142-4.
  38. Oslie, Pamalie Life Colors: What the Colors in Your Aura Reveal Novato, California:2000--New World Library Indigo Auras: Pages 161–174