AbstractThe perceptual colour attributes relevant for ordering unrelated, related, luminous, and nonluminous colours are different insome respects. Colour order systems for related nonluminous colours have been the most fully developed, but different systems depend on different types of experimental data, such as equalization of small perceptual differences, colour scaling, and some relationships to properties of the stimuli. The resulting systems have both similarities and differences, which are worthy of careful study. Chromatic adaptation is an important factor affecting and appearances of samples of colour order systems viewed in illuminants of various colours, and requires further study.