If you illuminate a surface on the wall with three light sources, a red, a green and a blue spot, you will get white light in the overlapping area of the spots.

This can also be explained using the spectrum of coloured LEDs shown below: If you combine the blue, green and red spectra, you get a fairly complete spectrum of all wavelengths, which is white light.

This principle of mixing light is called the RGB model (every monitor works on this principle). We speak of light colours.

All types of coloured light can be used for illumination. In industrial image processing, however, the use of LED lighting has become widely accepted. The emission spectra are typically quite sharply banded, which has practical advantages. Neighbouring wavelength ranges can be blocked very well by filters. The reverse is also of practical importance: Filters allow the coloured light to pass through and block any other extraneous light that might interfere.

The components to be tested are often coloured. The component is coloured using pigments or dyes.

In addition to colour filters that can be screwed onto a camera lens, printing inks also work according to this principle. Mix yellow and magenta and you get red, mix magenta and cyan and you get blue, and so on. The body does not reflect all of the incident light, but subtracts some of it. These are called body colours.

The colour model is called the CMY(K) model after the colours cyan, magenta and yellow. (The colour K=Key stands for black, which must be added during printing to obtain very dark colours and black tones.)