In early arts classes we all learn that there are three primary colours: red, blur and yellow. We can then create any other colour by mixing the three primary colours in different ratios. The colour green for example results from an equal mix of blue and yellow paint. This system was first developed by Sir Isaac Newton in 1666 when he published the first circular diagram of colours. Obviously the effects of mixed paints have been known before but Newton supplied the first rigorous theory of primaries and the colour circle which fairly precisely describes the result of any combination of primary colours. Clearly this system treats ‘colour’ as a property of paint or objects in general.


Usually at this point in our education no explanation is given why paints demonstrate this behaviour but we can readily verify the mixing theory experimentally. Mixing different ratios of primary colours allows us to reproduce all the colours found on the painters colour wheel. Unfortunately to very important ‘colours’ aren’t present on the colour wheel: black and white. No combination of the three artistic primary colours yields either a perfect black or white and in fact while we can at least produce something akin to brown-black sludge, we cannot ‘lighten’ our mix past the appearance of pure yellow (one of the primaries). To obtain a more complete theory of colour, we thus need to look a bit closer at the mechanism that causes an appearance of colour. The first step in this direction is to the physical properties of light, which is after all the link between objects and our eyes.