Colour only exists as a consequence of light; if there was no light, there would be no colour. Light is a form of electromagnetic energy which travels through space in waves. The unit of measurement of wavelength is the nanometre (nm) which is equal to 1 millionth of a metre. The electromagnetic spectrum is made up of waves of energy of varying length most of which are invisible to the human eye. Gamma rays, X-rays and ultraviolet rays at the short wavelength end and infrared, microwaves and radio waves at the long wavelength end of the spectrum can only be detected with equipment. Only the wavelengths between 380 nm and 750 nm can be seen and they are known collectively as the visible spectrum. The longest visible wavelengths are red and as they decrease in size become orange, yellow, green, blue, indigo and violet, the seven colours of the rainbow. When all the wavelengths are mixed together the result is white or colourless light.

In order to see colour, at least a little bit of light must be reflected off the surface of an object and be received by the eye. The light waves are received by the retina which has two kinds of cells that respond to them, rods around the sides which are sensitive to dim light and movement, and cones at the back which are sensitive to detail and colour. The information received by the retina is sent to the brain through the optic nerve for interpretation. The interpretation of colour depends on the amount and colour of the light absorbed or reflected by the surface of the object, so for example, when light strikes an object seen as yellow, it means that all the wavelengths in the light have been absorbed except for yellow which is reflected away.

Although each of the colours of the rainbow have their own wavelengths, some of them can be made by combining other different wavelengths together. Those which cannot be created in this way are known as primary colours which for light are red, blue and green. Mixing pairs of these colours together will make secondary colours, and subsequent combinations make all the other possible colours. Mixing colour as light will concern the design team in some circumstances such as buildings for the performing arts, special internal environments and the floodlighting of external facades, but is fundamental to the operation of film, television and computer monitors.

Of greater concern to the design of buildings is the mixing of colour as pigment for the selection of materials and colour schemes for the chosen finishes. Pigment is colouring matter used as paints or dyes and substances which give colour to materials, products and plants. When pigments are mixed together, the rules change because the primary colours are  red, yellow and blue. Adding pairs together create the secondary colours of orange, purple and green, and subsequent combinations make tertiary colours and so on, offering the possibility of an immense range of alternatives. A mixture of all the coloured pigments will ultimately create black. It has been suggested that the human eye can detect at least seven million different colour variations, which to all intents and purposes means there are limitless options when it comes to selection. This becomes clear by looking through a window into the garden at the trees, shrubs, flowers and grass, which can all be described as ‘green’. In fact, there are many different ‘greens’, some so dark that they are almost black, others quite light with yellows and white in them. The colours change depending on the brightness of reflected light, and how the light shines on the viewer, increasing and decreasing in intensity throughout the day.