This is a piece I did in college. We were learning about reflected light. I had a box with a red and blue side and a yellow bottom. Those are the primary colors. I set an egg in there and immediately it picked up the colors that surrounded it. I had a small paintbrush and put little dots of color as they appeared on the egg. This is the best example of reflected light I know of.
If two people stand next to each other and one has a green shirt and the other one is white, the white one will pick up the green. If the person in the white shirt stands outside on a sunny day, the part of his shirt in the shadow will pick up the color of the sky.