A student mixes ultramarine and cadmium yellow and gets a green darker than either tube. In the lighting rig she then aims a blue lamp and a yellow lamp at one patch of white wall and gets something paler than either lamp alone. Why do the two mixtures run in opposite directions?

A. Pigment subtractively filters light, whereas overlapping lamp beams add power.

Because the arithmetic is different. Two pigments each return a fraction of the incident light, and the mixture returns roughly the product of the fractions, which is always smaller than either. Two lamps each deliver power, and the powers add. Brightness of the source, surface reflectivity, and the choice of complements change nothing about that fundamental difference.