This article explores the relationship between the colour descriptors used in classical colour theory and machine vision, and the appearance of three-dimensional objects exhibiting geometric lighting effects. Various colour transformation equations are analyzed to determine their response to intensity changes, and their perceptual uniformity. It is suggested that a logarithmic chromaticity plane has advantages for the processing of images of three-dimensional scenes. The logarithmic function removes any intensity variation from the chromaticity metrics, and provides a chromaticity plane that is uniform with respect to hue, and reasonably uniform in chroma. Evidence of two types-mathematical and experimental-is given in support of this claim.