The best explanation of additive and subtractive mixtures I ever came across is written by Josef Albers’ in his book “Interaction of Color ” : with his exact words
There are 2 kinds of physical mixture:
a) direct mixture of projected light
b) indirect mixture of reflected light
a) color light, or direct color, probably is most familiar through its practical application in theater and advertising. The scientific analysis of the physical qualities ( wavelength etc ) is not the problem of the colorist ; it is the concern of the physicist. When he mixes his colors, he projects them on a screen, 1 on top of or overlapping the other. In any such mixture where there is overlapping, it will be obvious that every one of these mixtures is lighter than any of the mixture parents. By means of a prismatic lens, the physicist easily demonstrates that the color spectrum of the rainbow is a dispersion of the white sunlight. With this he proves also that the sum of all colors in light is white. This demonstrates an additive mixture.