Explanation

A Definition: -
"The sensation produced on the eye by rays of light when resolved...into different wavelengths"

The human eye is only sensitive to electromagnetic radiation within a narrow band of between 400 - 700 nanometers (nm), which we refer to as visible light. Normal white light contains radiation from the whole visible spectrum. When light is split into its various components, as can be achieved with a prism or indeed as seen in a rainbow, then the various colours can be seen. Light falls broadly into three bands or channels, blue 400-500nm, green 500-600nm and red 600-700nm, which are known as primary colours. Other colours, known as secondary colours, are achieved when light from the primary colours is mixed.

Dyes and Pigments

Simplistically a dye or pigment is a substance which is able to selectively absorb and reflect light at different wavelengths within the visible spectrum. The resulting observed colour is determined by which wavelengths are absorbed and reflected. If only light at 450 nm is reflected then a blue colour is produced, similarly if only light at 650 nm is reflected then a red colour is produced. The attached colour circle illustrates this principle in more detail. When light from the whole visible spectrum is absorbed and none reflected, then black is observed.

Colour circle