Most children know the mixture of blue and yellow paints looks green. But why? Does this also mean that the mixture of blue light and yellow light also looks green? If not, what color should the mixture of these lights look like and why?
Helmholtz was the first to explain why the mixture of yellow and blue paints looked green. To answer these questions, we should first understand why paints have different colors. A red paint on a white paper does not reflect extra red light than the paper without the red paint. Instead, the red paint absorbs all other colors than red. In general, the paints act as band-pass filters allowing only frequencies in a certain range to pass. On the other hand, a red light has most of its energy distributed in the frequency band of red light. In other words, the spectral distribution of a mixture of lights is the sum (or union) of the spectral distributions of the individual lights (a logic OR), while the spectral distribution of a mixture of paints is the intersection of the distributions of the individual paints (a logic AND). As a simple example, we easily see that the mixture of paints of all colors looks black, while the mixture of lights of all colors looks white.
Going back to the problem of yellow plus blue, if we are mixing paints, the intersection of the spectral distributions of these two paints (assuming they are wide enough) is in the frequency range for green. But if we are mixing lights, then the sum of the two distributions, if wide enough, may be as wide as to cover most of the visible visible frequency range so that the mixture may look light gray.
In the previous red-plus-green case, if we were mixing paints instead of lights, the mixture may look dark bluish, as both the red and green components of the white light are absorbed.