Figure 3.

Coloured crosses represent the material chromaticities (under skylight in the top panels and sunlight in the bottom panels), projected onto an equiluminant plane of MB-DKL space. These points are coded according to an observer’s classifications (as red or green in the left-hand plots and as yellow or blue in the right-hand plots). Black lines are examples of the intersection of fitted classification boundaries and the mean-luminance equiluminant plane. The chromaticity at which the red-green and blue-yellow classification boundaries intersect locates the chromaticity that is equally likely to be classified as red, green, yellow or blue. We infer this to be an observer’s achromatic point.