Matthew Poole and continuators on Leviticus 13:30

30 Then the priest shall see the plague, and behold, if it be in sight deeper than the skin, and there be in it a yellow thin hair , then the priest shall pronounce him unclean: it is a dry scall, even a leprosy upon the head, or beard.

The leprosy in the body turned the hair white, in the head or beard it turned it yellow. And if a mans hair was yellow before, this might easily be distinguished from the rest, either by the thinness or smallness of it, which is here noted, or by its peculiar kind of yellow, for there are divers kinds or degrees of the same colour manifestly differing one from another, as in green colours, &c.