This brief introductory chapter presents the topic of the book, impossible colours, and its aims, to give new accounts of colour experience and the mechanisms of the visual system that shape it. Impossible colours are forbidden by opponent processing, the prevailing account of how colour is encoded in the visual system, first proposed by Ewald Hering. Despite this, we can and do experience such colours. The book explores two kinds of these colours: (i) elementary colours beyond those proposed by Hering, and (ii) reddish greens and yellowish blues, which combine elementary colours in illicit ways on Hering’s account. Our experience of impossible colours requires us to rethink the accepted phenomenology of colour, which is shaped by the mechanisms proposed by Hering, and those mechanisms themselves. This chapter lays out this challenge and describes how the book will respond to it.