Metamer Mismatching Predicts Color Difference Ellipsoids

It is well known that color-discrimination thresholds vary throughout color space, as is easily observed from the familiar MacAdam ellipses plotted in chromaticity space. But why is this the case? Existing formulations of uniform color spaces (e.g., CIELAB, CIECAM02, CAM16-UCS) and their associated color-difference DE metrics are all models, not theories, based on fits to psychophysical data. While they are of great practical value, they provide no theoretical understanding as to why color discrimination varies as it does. In contrast, the hypothesis advanced and tested here is that the degree of color variability created by metamer mismatching is the primary (although not exclusive) factor underlying the variation in color-discrimination thresholds throughout color space. Not only is it interesting to understand the likely cause of the variation, but knowing the cause may foster the development of more accurate color difference metrics.