The theories which give the best account of the facts of colour vision and colour blindness are the Young-Helmholtz theory and Hering's theory. Both are trichromatic theories, and, apart from physiological or anatomical questions, both can, by proper choice of fundamentals, be made to give a good account of the main facts. The facts of one-eyed colour blindness show that, on the Young-Helmholtz theory, colour blindness must be regarded as due to fusion of at least two fundamental sensations, but the curves of one sensation determined by observations on different eyes differ considerably among themselves, and this indicates that a broader basis for the theory may be desirable. This may be sought for in a tetrachromatic theory, but any such theory must explain the possibility of trichromatic representation of all colours. A theory proposed assumes two pairs of complementary stimulations, say R,G1 and G2,V.