In order to judge whether a surface that one is looking at is white or grey, one needs to consider the intensity of the illumination. We here show that people do not simply use the maximal luminance in the light from the scene as a measure for the intensity of the illumination but also consider how luminance and chromaticity are associated. We suggest that they take into account that there are physical limitations to the luminance that reflecting surfaces can achieve at high chromatic saturation. These limitations arise because chromaticity is the result of surfaces selectively reflecting light of different wavelengths, so that the luminance of the illumination must be higher than that of the brightest patch in the scene if that patch is not white.