When lights of different colors shine on the same spot on a white surface, the light reflecting from that spot to your eyes is called an additive mixture because it is the sum of all the light.
At this exhibit, a red light, a blue light, and a green light are all shining on the screen. The screen looks white because these three colored lights stimulate the three types of cones in your eyes approximately equally, creating the sensation of white. We call this set of three colors—red, green, and blue—the additive primaries of light.
With these three lights you can make shadows of seven different colors—blue, red, green, black, cyan, magenta, and yellow—by blocking different combinations of lights: When you block two lights, you see a shadow of the third color—for example, block the red and green lights and you get a blue shadow. If you block only one of the lights, you get a shadow whose color is a mixture of the other two. Block the red light and the blue and green light mix to create cyan; block the green light and the red and blue light make magenta; block the blue light and red and green make yellow. If you block all three lights, you get a black shadow.
It may seem strange that a red light and a green light mix to make yellow light on a white screen. It just so happens that a particular mixture of red and green light stimulates the cones in your eyes exactly as much as they’re stimulated by yellow light—that is, by light from the yellow portion of the rainbow—so your eye can't tell the difference.
As you move in front of the wall, your body can block one, two, or all three of the lights, resulting in shadows of various colors. (click diagram to enlarge)
