VISUAL sensitivity is controlled by at least two distinct types of adaptation : light adaptation adjusts sensitivity to the mean luminance and colour in the stimulus(1), and contrast adaptation adjusts sensitivity to the variations in luminance and colour(2-5). Light adaptation is thought to be important in maintaining the perceived colour of objects despite changes in illumination (’colour constancy’), compensating for the mean changes in the light reflected from scenes under different illuminants(6). But for naturalistic colour signals, we show here that changes in an illuminant can also alter colour contrasts in images (how colours are distributed around the mean) enough to alter the state of contrast adaptation. Thus perceived colour under different illuminants may also be noticeably influenced by contrast adaptation.