This one always stumped me. (well once)
Click to see full-size image. |

Which is Knight is the white one and which Knight is on the white square?
don't get this one. Presumably it's the one that looks like it's black and on the black square rather than vice versa, but don't know why!
They're the same colour. Even though the light from the image comes to your eyes and largely influences what you see, there are top down elements as well, like the expectation of what shade you should be seeing in shadow. I think.
If you make two small holes in a bit of paper and cover the image but with the two relevant squares showing through the holes, you can see they're the same colour.
If in doubt about these things, always best just to put them to RSQ, let the masses decide!