Robert Krulwich points out that there is no pink in the rainbow, linking to a youtube video:
Saying that pink isn’t a color is a little silly, though. The space of colors that humans perceive is three-dimensional. Start with an actual light source, which has a continuous spectrum; human vision roughly projects from the space of possible spectra to a three-dimensional space, each dimension corresponding to one of the three types of cones. The “pure” colors (single wavelengths) correspond to a two-dimensional manifold in that space — one dimension for hue, one for brightness. Just because you wouldn’t call anything in that two-dimensional space “pink”, that means pink isn’t a real color?
There must be some aliens, somewhere out there in space, that have yellow-sensitive cones as well and are offended that we don’t think yeen and grellow are real colors.
(hat tip: LA)