Unique Hues and Colour Experience

In this Handbook entry, I review how colour similarity spaces are constructed, first for physical sources of colour and secondly for colour as it is perceptually experienced. The unique hues are features of one of the latter constructions, due initially to Hering and formalized in the Swedish Natural Colour System. I review the evidence for a physiological basis for the unique hues. Finally, I argue that Tye's realist approach to the unique hues is a mistake.