Color of the Fur coat of Polar bears
The fur coats of polar bears are in fact clear tubes - not white fur as is often believed. It is their skin that is black, to absorb the most warmth from the sun (and the clear tubes allow the majority of the light to pass through to the skin).

I recently watched the BBC's Frozen Planet TV series (with David Attenborough), where I heard the above answer. If you haven't already, I recommend watching it.