While there are no surfaces, paints or lights that are reddish green or yellowish blue, many of us, I show here, can experience these colours. This chapter includes examples of stimuli that are reported to produce experiences of reddish green and yellowish. This is especially striking in the context of the philosophy and science of colour, because these colour experiences are forbidden by the dominant theories of colour: Hering’s account of colour experience, and the opponent processing theory.