Everybody has their 'favorite' colours and there are no ends of books available 'giving hidden insights into your personality if your favorite colour is purple’. It is however proven that certain colours have predictable effects on people, and this is used in advertising, environmental planning and communication.

Red stimulates the flow of adrenalin, which increases muscular tension, blood pressure, respiration and body heat. What this means is that red will make us feel edgy and restless. It is no coincidence that 'red for danger' is an accepted phrase, or that when we are angry we 'see red’. This physical effect is used in warning and prohibition signs - for example ’Stop' on the traffic lights, 'no entry' signs, hazard warnings.

Green is the Opposite colour to red (see Hue Circle), and whereas red in stimulating and irritating the psychological effect of green is restful and soothing. It is for this reason that green is used to convey ‘safe ‘messages - 'go' at the traffic lights, first aid post. Green is also commonly used in hospitals; if you are at ease psychologically, then the physical healing will take place more readily.

Certain colour combinations are also used to convey information, as well, notably black and yellow. These colours are often used to warn us of dangers, like the 'Highly flammable' grenade symbol, or the black skull and crossbones on a yellow background which indicates Dangerous Chemicals. These colours have significance for creatures other than humans, as well. Birds, which are blessed with good colour vision, learned many millennia ago that if they ate bees or wasps then they suffered very unpleasant consequences. Insects also found that if they weren't coloured black and yellow, then they stood a fairly high chance of getting eaten. So in the interests of self-preservation birds don't eat black and yellow insects and many varieties of moths and flies have evolved black and yellow stripes as protective colouring.

It is a bit strange to think that we describe grass as green, when that is the colour which the grass is rejecting. In order to match a given colour, a blend of two or more pigments is likely to be used. If one of these is white, this will dilute the effect of coloured pigments, less intense or paler.

When two coloured pigments are mixed, they will both be absorbing a considerable amount of light, so mixing them will increase the amount of light absorbed, because we are taking away light from the visible spectrum by mixing  coloured pigments. This is known as subtractive colour mixing.

Subtractive colour mixing is the principle used by the paint maker, for a given colour. If we mix a blue pigment and a yellow pigment, we do not get white, we get green. This is because the blue pigment absorbs practically all of the orange and most of the red and yellow wavelengths from the white light falling on it, the yellow pigment absorbs most of the violet, the remainder of the red and nearly all the blue.

If we look at the six principle colours, we see that green is not absorbed by either pigment, so its appearance is green. Both pigments are extremely fine in particle size and are uniformly distributed over the area covered by the mixture. Light falling on that area passes through a double filter and the only wavelengths returned are those not absorbed. If both pigments are dull (absorb strongly), the green will be even duller because less light energy will reach the eye.

If red and green pigments are mixed together, absorption of all frequencies takes place so the result should be black, but in fact, not all frequencies are completely absorbed and the result is a grey colour.

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In the pigment circle a number of pigments are listed which do not necessarily correspond to pure colours, but may lie between them. If two pigments are mixed by drawing a line between the pigments e.g. red and blue can produce violet, the closer the line is to the center of the circle, the darker the colour will be. Similarly, the closer to the edge, the cleaner will be the colour (non-complementary colours).

To adjust the hue (the colour within a colour like the redness within a blue colour), the opposite colour within the circle may be added, e.g. add green to reduce the redness or red to reduce the greenness within a colour.