The second series of experiments sought to address the issue of colour constancy. Colour constancy is the perceptual phenomenon of apparent perceived colour invariance of a surface under different illuminations. For example, the ability to correctly distinguish the colour of an object when illuminated by yellow sodium street lighting, as if it were viewed in normal daylight. Coloured illumination was used to add a hue to the display and to an adapting surround in the experimental chamber. Subjects were asked to repeat the discrimination task of experiment 1 under a different coloured illumination. Figure 2 shows typical results for a green illuminant. Here the distinct arrangement of the category boundaries is preserved but shifted in the direction of the illuminant. Similar results were found for all cases of illumination of different colours, i.e. red illumination shifted the categories towards red, blue towards blue and so forth. The relative category borders do not change.

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