Rashi on Leviticus 13:29/רש"י על ויקרא י״ג:כ״ט

1בראש או בזקן UPON THE HEAD OR UPON THE BEARD — Scripture intends to make a difference between a plague which is on a place where hair grows and a plague which is on a place where flesh is (already mentioned v. 38) — that as regards this (that on the flesh) its symptom of uncleanness consists in white hair, and as regards the other its symptom consists in gold-coloured hair (Sifra, Braita d'Rabbi Yishmael 14).