This book is about impossible colours. Impossible colours earn their name because they are forbidden by opponent processing, the orthodox scientific account of how the visual system encodes colour. Yet there are good reasons to think we can and do see such colours. This book investigates two different kinds of impossible colours. First, there are reddish greens and yellowish blues, which are often thought to be impossible to see or even conceive. A range of evidence is given here that shows this to be false – including colour figures that might allow the reader to see these colours themselves. The second kind of impossible colour is entirely familiar, indeed mundane, and includes colours such as orange, purple and brown. The book assembles evidence showing that these colours are experienced as phenomenologically elementary, a fact that contradicts opponent processing theory. This book also provides a new account of colour experience, to which these two kinds of colours provide the keys, and explores the deep implications for scientific and philosophical thinking which follow. For science, impossible colours call into question the opponent process theory. For philosophy they give new perspectives on long-contested questions. Are colours characteristics of the external world, which our visual systems grant us direct experiential access to? Or are they generated in some way by the visual system, an illusion we project onto the external world?