The paint alone, without the light: the right ball is painted dark on top and light underneath.
Fooled by depth
Countershading
Two balls in light from above. Are they the same shape?
Yes: both are round. The right one is painted darker on top and lighter underneath, which cancels the light, so it looks flat.
Why? Your brain reads a round shape from its shading: lit on top, darker underneath. Take the shading away and the shape goes flat. Many animals are colored this way, dark backs and pale bellies, so they are hard to see. Abbott Thayer noticed it.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)