What is colour? How does colour vision work? Why is the sky blue? What is the colour spectrum? The answers to these and many other related questions about colour physics are each provided in a short and easy-to-understand form. Will delight and entertain colour professionals and curious members of the public.What is additive colour mixing? Additive colour mixing refers to the mixing of different (coloured) lights and can be easily demonstrated by the superposition of lights (known as primaries) on a white projection screen. When this is done using red, green, and blue primaries, the colours yellow, cyan, and magenta are produced where two of the primaries overlap. Where all three primaries overlap the sensation of white is produced if the spectral distributions and intensities of the three primaries are carefully chosen. Additivity is not a special property of any particular set of three primaries. The range of colours that can be matched with any three primaries is called the gamut of those primaries. One often reads that the primaries are pure and cannot be matched from other colours.

This is not true. If one uses three primaries such as red, green and blue it is true that none of the primaries can be matched by mixtures of the other two or by mixtures of any other colours in the gamut of the primary system. However, one could select colours outside the gamut of the system which when mixed together cold match the primaries. It turns out that no three real primaries can be chosen so that their gamut includes all possible colours. If the primaries are chosen to be red, green, and blue, however, a very large number of colours can be matched. Red, green, and blue are therefore usually the colours of the primaries in an additive colour reproduction system such as colour television.