Indigo is a frequency range of visible light, from 440 to 420 nanometers in wavelength, placing it between blue and violet. If your browser supports color display, the following patch should be indigo:
Indigo does not fit into the "primary" (additive) or "secondary" (produced) categories of colors. It was named and defined by Isaac Newton when he divided up the visible 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.