Leviticus 13:35

And in the seventh day the priest shall look on the scall: and, behold, if the scall be not spread in the skin, nor be in sight deeper than the skin; then the priest shall pronounce him clean: and he shall wash his clothes, and be clean.

Then the priest shall look on him: and, behold, if the scall be spread in the skin, the priest shall not seek for yellow hair; he is unclean.