- This article is about the color. For other meanings, see indigo (disambiguation).
Indigo is the color of light between 440 to 420 nanometres in wavelength, placing it between blue and violet. Like many other colors (orange and violet are the most well-known), it gets its name from an object in the natural world - the plant named indigo once used for dyeing cloth.
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 named seven colors specifically to link them with the (known) planets, days of the week, notes in the octave, and other lists that had seven items.
The human eye is relatively insensitive to indigo's frequencies, and some otherwise well-sighted people cannot distinguish indigo from 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.
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