[petercooper]

It's really striking if you have a projector. Before you turn on a projector in a room that isn't dark, a white wall looks pretty white. Once the projector is on, that "pretty white" wall is now playing the role of black in the projected scene.

[greenimpala]

That's almost so obvious that I never thought of that :D

[grishka]

And the color brown doesn't exist on its own. It's orange with context.