Neither light can get to region C, so it shows up black. In region A, the red light is shining but the green is blocked by the card. Just the opposite is true for region B. About mixing, pigment is different from light. In such situations, colors subtract . In pigments, for example, blue and yellow make green.
Lights are different.
When you put more light in, the colors add . The more colors you put in, the closer you get to white, the mixture of all colors. With light, though it seems weird, red and green make yellow. The reason for the colored shadows may be too hard for very young children, but they can still reason about mixing colors and see the patterns that emerge. The Exploratorium has more material on colored shadows .