In the theoretical section of the present paper, we develop our view of the roles played by the perceptual criteria: indistinguishably equal, neither blue nor yellow, neither green nor red, and heterochromatically equally bright. These criteria constitute a vectorial opponent-colour space, a concept used throughout the paper. Within this framework, two new theorems on psychophysical opponent-colour channels are stated. In the experimental section, the perceptual criteria are applied to the colour vision of an (extreme) deuteranomalous male. A quantitative perceptual description of his deuteranomaly is developed, the main constituents of which are measured deuteranomalous colour matching functions, the deuteranomalous opponent-colour functions derived from them and taking quantitative account of the Abney effect.