To demonstration the principle of Color Complements.
Take a sheet of colored paper, a small square of grey cardboard, and a sheet of wax paper. Then place the cardboard square on the colored paper (a bright red or green color is ideal). Then cover both paper and square with the wax paper folded once, twice, or three times, depending on its thickness. When seen through the transparent paper, the square will appear strongly tinted with a color complementary to the color of the paper. That is, it will seem light green if red paper is used or, pinkish if green paper is used. This effect is related to the fact that shadows of colored objects acquire a tint of the complementary color.