It is hypothesized that colour vision and opponent processing of colour signals in the visual system evolved as a means of overcoming the extremely unfavourable lighting conditions in the natural environment of early vertebrates. The significant flicker of illumination inherent in the shallow-water environment complicated the visual process in the achromatic case, in particular preventing early detection of enemies. The presence of two spectral classes of photoreceptors and opponent interaction of their signals at a subsequent retinal level allowed elimination of the flicker from the retinal image. This new visual function provided certain advantages concerning reaction times and favoured survival. This assumption explains why the building blocks for colour vision arose so early, i.e. just after the active predatory lifestyle was mastered. The principal functions of colour vision inherent in extant animals required a more complex neural machinery for colour processing and evolved later as the result of a change in visual function favouring colour vision.