One can see spectral indigo by looking at the reflection of a fluorescent tube on a non-recorded 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.
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 swatched 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.
In an RGB color space, spectral indigo and violet must be approximated by purples, that 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 was defined as a spectral color by Sir Isaac Newton when he divided up the optical spectrum, which has a continuum of wavelengths. 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.
The human eye is relatively insensitive to hue changes in the wavelengths between blue and violet, where Newton defined indigo to be; most individuals do not distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, hold 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. Others continue to accept it, 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.
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.
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)