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'Indigo' is the color on the spectrum between about 450 and 420 nm in wavelength, placing it between blue and violet. Color scientists do not usually recognize indigo as a significant color category, and generally classify wavelengths shorter than about 450 nm as violet. Measuring Color, J. W. G. Hunt, , , Ellis Horwood Ltd, 1980,
Indigo and violet are different from purple, which cannot be seen on the electromagnetic spectrum but can be achieved by mixing blue light with red light.
One can see spectral indigo by looking at the reflection of a fluorescent tube in a non-recordable compact disc. This works because the CD functions as a diffraction grating, and a fluorescent lamp generally has a peak at 435.833 nm (from mercury), as is visible on the fluorescent lamp spectrum.

Contents

Distinction between four shades of indigo

Electric indigo

Deep indigo (web color blue violet)

Pigment indigo (web color indigo)

Indigo dye

Indigo in human culture

See also

References

Distinction between four shades of indigo

Like many other colors (orange 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 an approximation of ''spectrum indigo''. This is the brightest color indigo that can be approximated on a computer screen—it is the color between the web color blue and the color electric violet.
The web color ''blue violet'' or ''deep indigo'' is a shade 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 than either spectrum indigo or pigment indigo. This is the actual color of the dye from the indigo plant when switched onto raw fabric. A vat full of this dye is a darker color, approximating the web color Midnight Blue.
When specifying the color ''indigo'', it is necessary to indicate which particular one of these four major shades of indigo you are referring to.

Electric indigo

In an RGB color space, spectral indigo and violet must be approximated by purples, this is, by mixing a little red with a lot of blue. Spectral indigo is closely approximated by the color 'electric indigo'. This sample was taken directly from the CIE chromaticity diagram opposite the 430 nanometer line. It is much brighter than the pigment indigo reproduced below. Spectrum Indigo fits nicely between spectrum violet and spectrum blue as can be seen in the color bands displayed below. It is impossible to represent spectrum indigo exactly on a computer screen, because true spectrum indigo is outside the color triangle or gamut of the RGB color space defined by the monitor primaries.
Indigo is neither an additive primary color nor a subtractive primary color. It was named and defined by Isaac Newton when he divided up the optical spectrum (which is a continuum of frequencies). He specifically named seven colors primarily to match the seven notes of a western major scale, because he believed sound and light were physically similar, but also to link colors with the (known) planets, days of the week, and other lists that had seven items.
Humans do not tend to recognize indigo as a separate hue category between blue and violet. For this reason, some commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet. Color scientists typically divide the spectrum at about 450 nm between violet and blue, with no indigo.^[1]
Others continue to accept it ^[2] as it has been accepted traditionally as one of Newton's named colors of the spectrum along with red, orange, yellow, green, blue, and violet.

Deep indigo (web color blue violet)

At right is displayed the web color 'blue violet', a color intermediate in brightness between electric indigo and pigment indigo. This color is also called 'deep indigo'.

Pigment indigo (web color indigo)

The color box at right displays the web color Indigo which is equivalent to 'pigment indigo', the color indigo as it would be reproduced by artists' paints as opposed to the brighter indigo above (electric indigo) that it is possible to reproduce on a computer screen.
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 a lot brighter because they are produced from light instead of pigment.
Pigment indigo (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.

Indigo dye

At right is displayed the color 'indigo dye', an approximation of the color of a swatch of indigo dye.
Sample of Indigo Dye color: ISCC-NBS Dictionary of Color Names (1955)--Color Sample of Indigo Dye (Indigo color sample #179)

Indigo in human culture

'Food'

★ The outer skin of many varieties of eggplant is colored pigment indigo.

★ So called "purple" carrots and potatoes are actually colored pigment indigo.
'Midwifery'

★ In the ancient Maya civilization indigo was the one color that was recognized for its soothing effect on pregnant women and their unborn children. Pregnant women often wore this color in order to protect themselves and their child from danger and to ensure a safe and successful delivery.
'Music'

★ The Indigo Girls are a folk rock group.
'New Age Philosophy'

★ The color electric indigo is used to symbolically represent the sixth chakra (called Ajna), which is said to include the third eye. This ''chakra'' is believed to be related to intuition and gnosis (spiritual knowledge).
'Parapsychology'

★ Psychics who claim to be able to observe the aura with their third eye generally associate indigo, in auras, with an interest in religion or with intense spirituality and intuition. ^[3] Indigo children are said to have predominately indigo auras. People with indigo auras are said to be in occupations such as computer analyst, animal caretaker, and counselor.^[4]
'Sociology'

★ Indigo was the color worn by the Dalits (Untouchables) in India before their pariah status was made illegal. If those of the higher castes touched any indigo object, they had to perform a ritual expiation. ^[5]
'Video Games'

★ The color most associated with the Nintendo Gamecube is a purplish indigo, officially deemed indigo.

See also

★ List of colors

★ Indigo dye

★ Indiglo, a brand name for a method of electroluminescence technology

References

1. Fundamentals of Atmospheric Radiation, Craig F. Bohren and Eugene E. Clothiaux, , , Wiley-VCH, 2006,
2. Tansley, David V. ''Subtle Body: Essence and Shadow'' New York: 1984 Avon (Art and Cosmos series--Jill Purce, Editor) It is pointed out that New Age artists and philosophers accept indigo as a spectrum color because it is used to represent one of the seven chakras (the sixth) of the subtle body.
3. 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.
4. Oslie, Pamalie ''Life Colors: What the Colors in Your Aura Reveal'' Novato, California:2000--New World Library Indigo Auras: Pages 161-174
5. Varichon, Anne ''Colors:What They Mean and How to Make Them'' New York:2006 Abrams Page 171