Polar bears have translucent fur, which means that it only appears to be white because its actively reflecting any visible light. Their skin is actually black, and their hairs are hollow. The long, coarse guard hairs, which protect the plush thick undercoat, are hollow and transparent, while the thinner hairs of the undercoat are not hollow, but they, like the guard hairs, are colorless. When they are just cubs, polar bears have brown fur but as they grow and mature, they develop the white fur they’re easily recognized by.