Ask anyone what color a polar bear is and they will exclaim "white", but their fur is actually transparent and holds no color. Their fur is a series of clear, hollow, translucent tubes and it only appears white because it reflects visible light. Polar bear skin, surprisingly however, is black!
They are almost entirely invisible to infrared photography. The polar bears' layers of fur and blubber trap their body heat well below their skin, leaving the outer layer to be roughly the same temperature as the snow around them.
To humans and other creatures that see only in visible light, polar bears nearly blend in seamlessly with their snowy environment. However, reindeer have outsmarted the polar bear's tricky fur by evolving a visual system than can see in ultraviolet light, which means polar bears stand out like a sore thumb against their icy white backdrop.