I've always bought my bucktail, it sounds like you've got one that is off white - and you want to get it pure white.
I doubt you would be able to do that by washing it. You could dye it white or maybe there is a way to bleach it white.
I've been told to use borax if the skin hasn't dried or gets funky on you.
After the tail has cured, so there is "no hair slipping"....... you can dye them any color you want using a permanent dye....... again when you take them out of the solution, use a hair drier and a brush or comb to make them purty..................