Synaesthetes persistently perceive certain stimuli as systematically accompanied by illusory colours, even though they know those colours to be illusory. This appears to contrast with cases where a subject's colour vision adapts to systematic distortions caused by wearing coloured goggles. Given that each case involves longstanding systematic distortion of colour perception that the subjects recognize as such, how can a theory of colour perception explain the fact that perceptual adaptation occurs in one case but not the other? I argue that these cases and the relationship between them can be made sense of in light of an existing view of colour perception. Understanding colours as ways in which objects and surfaces modify light, perceived through grasping patterns and variations in colour appearances, provides a framework from which the cases and their apparent disanalogy can be predicted and explained. This theory's ability to accommodate these cases constitutes further empirical evidence in its favour.