Anyone interested in how color and value contrasts work in an image has got to check out the cover story of the MAY-JUNE 2002 issue of American Scientist. The pictures are amazing, starting with the cover which shows a cube covered with colored dots. The light is from the upper left. The brightly lit top of the cube is covered with yellow dots with a dark (almosy black) blue dot in the center. The side of the cube in shadow is covered with dark gray/black dots with a glowing, almost electric blue dot in the center. The amazing thing is that both blue dots are acually the same color! I had to use two cards with punched holes in them to block out everything but the blue dots before I could admit that this was true. Inside there are more images showing that the exact same shade of gray can be used to represent a white object in shadow or a dark (even black) object in bright light.
Oh, why don't I just show you the cover: