The best test for Hg (mercury) is the Hair analysis, and Doctor's Data are a good lab in this regard. You can do either a head hair or a pubic hair, head hair is "preferred" because much of the research (and reference ranges) are based on head hair (from the nape of the neck). But in saying that, pubic hair is better in terms of finding out a longer history of heavy metals. Pubic hair is cut much less often and is not exposed to the environment. I request pubic hair sample from a patient if I am highly suspicious of heavy metals for example if they are a jeweler, plumber, welder, electrician, etc.
Not sure about the white hair thing and heavy metal though, I'd probably work with such a patient with digestive enzymes and a whole-foods multi vitamin to see if I could make a change to their digestive function.