What gives observable light its colors?
Three different types of cones in the eye are sensitive to three different, contiguous and slightly overlapping, sections of the visible spectrum. The nerve signals from these three different types of cones are interpreted by different sections of the brain as cyan, magenta, and yellow, from which all the discernible colours are composed. This forms the familiar Colour Triangle.

The sides of the Colour Triangle bulge slightly (I believe) because of the slight overlap in the sensitivity ranges of the three cone types.

Colour blindness occurs when someone is born with only two (or rarely one or none) different types of cones, instead of the usual three. This causes an inability to detect one of the thee sub-spectra.