Although our distant ancestors once had the same visual sensor arrangement as extant birds and reptiles, colour vision in most mammals is very limited; essentially reduced to a single dimension of information produced by a pair of colour sensors, each with a fairly broad spectral response. Humans belong to a small subset of mammals that have re-developed some of the ability that was lost by their distant ancestors. They have done this by stretching the natural variation of the light absorbing pigments in one of the visual sensors, thereby adapting it toward a third colour sensor. It is generally assumed that due to the differences and limitations of this solution that colour perception in humans is fundamentally different to that of our distant relations; animals which have been able to continually extend and refine their colour vision over a period of more than two hundred million years. However, over that extended period of time the fundamentals of their colour vision have remained largely unchanged; a system of four visual sensors, each with a specific colour filter that systematically narrows the spectral response. In this paper we demonstrate that a three-sensor system with a broad spectral response (similar in spectral profile to that of the human colour sensors) can in certain conditions be equivalent to a four-sensor system (broadly similar to bird and reptile colour sensors). This suggests that human colour perception (sometimes referred to as a 'trichromatic' system) may be broadly equivalent to colour perception found in a variety of other animals such as birds or reptiles (whose colour vision is generally referred to as 'tetrachromatic'). The difference between a sensor system that uses four spectrally narrow colour sensors arranged into two opponent pairs and a three-sensor system is that the former relies on a simple difference measurement whereas the later requires the use of complex trigonometric functions - which are at least two orders of magnitude greater in computational complexity.