Why does mixing colored light differ from mixing paint?

A screen can make a bright yellow-looking patch using red and green light, while mixing red and green paints usually gives a very different result. The difference begins with what reaches the eye. Light sources add contributions; pigments alter light that arrives from another source.

With additive light, red and green contributions can reach the same part of the visual field together. The visual system responds to the combined light in a way that can appear yellow. Neither source has to become a yellow source. A perceived color describes the response to the light reaching the eye, not necessarily one newly created wavelength.

The illustration uses ideal red, green and blue channels at equal normalized full strength, chosen and calibrated to give the stated reference white. Red plus green gives yellow, red plus blue gives magenta, and green plus blue gives cyan in this model. All three give white. Changing the relative channel strengths changes the result.

Paint follows another route. Pigment particles absorb selected parts of the illuminating light and scatter some of what remains. Mixing pigments changes which light survives these interactions and returns toward the eye. It combines light-removing materials rather than simply adding colored beams, so the additive RGB rules do not predict a mixture of paints with the same color names.

A screen and a painted surface can sometimes look similar while producing their light by very different mechanisms. Particular lamps, pigments, illumination and viewing conditions affect the match. Following the light from source to material to eye explains why color mixing needs a stated physical situation as well as color names.