In Colour, Victoria Finlay writes that white, as a colour has been made from a variety of sources:
It can come from chalk or zinc, barium or rice, or from little fossilised sea creatures in limestone graves. The Dutch artist Jan Vermeer even made some of his luminescent whites with a recipe that included alabaster and quartz - in lumps that took the light reflected into the painting and made it dance.
...The greatest of the whites, and certainly the cruellest, is made of lead.
White paint is white because it reflects most light rays away from it...
detail from here