Explanation

A red object appears red because it reflects mostly red light back to your eyes. All the other colors of visible light are absorbed into the object. When you put a red filter over the hole in the shoe box, it filters out all the colors in the white light except for red. When this red light strikes the green gumball, it contains almost no green light for the gumball to reflect, so the gumball looks black. The same is true for the blue gumball. You can’t even tell the difference between green and blue. Almost all the light that strikes the red gumball gets reflected. Someone who looks in your red-filtered box containing a red and a blue gumball and tells you what colors they are will probably guess that they are white and black. This is because we see objects that reflect a lot of light as white and objects that absorb light as black.