Abstract
I explore an understanding of colour appearance predicated on the brain's mapping sensory inputs into discrete categories conveying the maximum Shannon information about the input. Under
sufficiently high (but not infinite) signal-to-noise ratio, when an input ensemble contains the usually large dynamic range, an information maximizing mapping from the contrast-gain-controlled photo-receptor inputs to, eg six, categories typically carves the input space into regions that correspond
to the perception of white, black, red, green, blue, and yellow colour categories. This input-to-
category mapping corresponds to another mapping from surface reflectance to category of colour appearance. Illumination changes that sufficiently preserve signal-to-noise can alter the input-to-
category mapping but leave the reflectance-to-category mapping almost unchanged, achieving colour constancy. This hypothesis of informationally optimal colour boundaries, when applied to small input ensembles made of inputs from a single or a part of a scene, can account for various
colour illusions and, under achromatic inputs, typical phenomena in lightness perception. It does not at present accommodate the spatial configuration factors that influence colour/lightness
appearance.
