Mixing coloured pigments
One of the first things that a student of painting learns is that green paint can be made by mixing yellow and blue paint.
This would not be possible if the paints available were pure yellow and pure blue. The success of this method of making green paint depends on the fact that the pigments in common use are impure colours.
Yellow paint is a compound yellow so that, when illuminated by white light, it reflects red, yellow, and green light and absorbs the blue. Similarly, blue paint is not a pure colour: it reflects blue and green and absorbs red and yellow. When the two paints are mixed, then between them they absorb red, yellow, and blue. The only colour they both reflect is green. Consequently, the mixture looks green. This process is called colour mixing by subtraction to distinguish it from the effect of mixing coloured lights by reflection from a white surface which is called colour mixing by addition.