Skin and fur get their colour because the tyrosinase protein helps skin cells manufacture melanin. A genetic defect in tyrosinase leads to albinism because melanin cannot be made - either cannot be made at all, or cannot be made properly. This is "c-locus albinism" and it also affects the visual pathways, sometimes resulting in crossed eyes because the nerves from the eyes connect up in an abnormal way. In domestic cats, there are several albino mutations: Burmese sepia (changes black to brown, orange to yellow), Siamese colourpoint (colour is restricted to head/legs/tail), blue-eyed albino and pink-eyed albino. There is also the "Inhibitor" (Chinchilla) gene that prevents deposition of pigment on the hair shaft, but allows pigment at the hair tips - this is the mutation found in white Bengal tigers.
The normal colour of lions varies from sandy-golden through to tawny-brown with the male's mane being dark brown or black in some subspecies. Faded spots and rosettes may be visible under some lighting conditions. According to Roy Robinson, noted feline geneticist, white lions have either the chinchilla or acromelanism mutation. Some older literature mistakenly referred to chinchilla as a form of albino. The appearance of blond and white lions (colloquially called leucism) means that colour variations in lions are probably more common than originally thought. The high mortality rate in lion cubs means that those colour variations haven't been observed in the past. White lions have survived due to human intervention.
Though not as common as white tigers, white lions are now being found in more and more zoos and may well be the current "must have" big cat. Their value as attractions may well send them down the same path as the white Bengal tiger: mass production, inbreeding and indiscriminate crossing with other subspecies e.g. to produce larger, showier manes for circus acts. Already they have been bred in bulk with no regard for health and used in canned hunts. If crossed with stripeless white tigresses, white or blond ligers would result - something almost certain to happen one day because the huge size of ligers makes them attractive exhibits.
For those interested in human genetics, albinism and leucism are found in humans; in one striking case of human leucism, a young girl of African origin presented with milk white skin, normally pigmented [brown] eyes and golden hair [personal observation, November 2003]