Mrrzy: I'd thought of that point (that color can be defined by our perception of it), but I wanted to clarify that there is no white pigment in the bear's fur and that the bear's coloration appears differently depending on the circumstances. So we can agree, I think, that a polar bear "is" white sometimes.

So to answer the question "What color is the bear?" in the riddle by saying "white", we'd have to assume that the conditions were such that he would appear to be white. Again, one of those assumed standard conditions that irritate me about riddles!!!