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This is about the colour Indigo. There is also the place Indigo, Victoria in Australia.

Indigo is a frequency range of visible light, from 440 to 420 nanometers in wavelength, placing it between blue and violet. On a browser that supports visual formatting in Cascading Style Sheets, the following box should appear in this color:

Indigo does not fit into the "primary" (additive) or "secondary" (produced from equal combinations of primaries) categories of colors. It was named and defined by Isaac Newton when he divided up the optical spectrum (which is of course 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 frequencies, and some otherwise well-sighted people cannot distinguish indigo from blue and violet.

See also Indigo dye, Indigo plant, Indigo Books and Music