Depends. I don’t know exactly, but yes, bleaching can be used. White uterine vellum is made from calf, generally, so doesn’t need bleaching, just careful preparation.
Pigskin vellum, when carefully tanned to such, is a very pale beige, assuming a pale pig. A darker pig is darker tan, even to a khaki, sometimes with brown splotches.
A parcemenaius’ skill was measured in how he cut the skin to maximize the useful area. An illuminator’s in makeing use of the extant skin blotches.
Also, some are treated with a white chalk glaze made with VERY fine powdered chalk. That wash both provides a near-white surface, but also slows ink absorption, and a better writing texture.