I think I finally figured out the reason why Shiro had a white tuft of hair.

When he was first captured by the Galra, they may have done some experiments involving quintessence and used some of it on Shiro. It wasn't a whole lot, but it was enough just so a part of his hair turned white.

I know it's ridiculously short but I don't really know any other way to explain it.

Just can't believe it took me six seasons to figure this out though.

~Webs