In science, the primary colours are red, green and blue. However, in art (e.g. painting), the primary colours are red, yellow and blue. Why the difference?

In science, we are interested in how light of different colours combine to form other colours in the spectrum. It has been established scientifically that for the human eye, light of any colour can be produced by combining red, green and blue light of various intensities.

Use a magnifying glass and look closely at a colour TV. You will notice that the screen is filled with tiny red, green and blue rectangles. These rectangles are activated to various degrees so that they glow with varying brightness. The human eye blends them together, resulting in the different hues that we see.

In art, the primary colours are red, yellow and blue. To be exact, they should be magenta, yellow and cyan respectively. Here, we are dealing with pigments. A pigment or a coloured object absorbs all the light except the light of its own colour. Thus, a yellow object emits only yellow light and absorbs all other light.

It so happens that magenta (i.e. nearly red), yellow and cyan (i.e. nearly blue) pigments best absorb green, blue and red light respectively. So, mixing yellow and cyan pigment, for instance, produces a green pigment, because blue and red light are absorbed, and only green light is released.

Mixing various amounts of these three pigments produces all the colours you want on the palette.