This paper presents a systematic analysis of the role of opponent type processing in colour vision and the relation between opponent type colour transformations and the initial three colour mechanisms. It is shown that efficient information transmission is achieved by a transformation of the initial three colour mechanisms into an achromatic and two opponent chromatic channels. The derivation of the transformation is dependent solely on criteria from information theory. Thus it provides a logical rationale reconciling opponent type processing as an optimal necessary step after the initial three colour mechanisms, unifying respectively the Hering and Young--Helmholtz approaches to colour vision. The effects of chromatic adaptation on the spectral response of the achromatic and two chromatic channels are discussed from the point of view of information theory. It is argued that adaptation serves as a dynamic readjustment of these responses, necessary to meet criteria of efficient colour information transmission. The results are confronted with empirical observations to test the principles of the theory and the relation to other theories is discussed. Within the same framework the issue of trichromacy is discussed. It is argued that a broad class of typical colour spectra can effectively be represented by three significant degrees of freedom that make up a trichromatic system.