Rashi on Leviticus 13:3/רש"י על ויקרא י״ג:ג׳

1הפך לבן [THE HAIR …] IS TURNED WHITE — This means that at first it was dark, and it turned white within the plague. The smallest number of hairs implied by the term שער is two (Sifra, Tazria Parashat Nega'im, Chapter 2 2).

2עמק מעור בשרו [THE PLAGUE] IS DEEPER THAN THE SKIN OF HIS FLESH — Every white colour is deep (in contrast to a dark colour surrounding it) just as the colour of anything illuminated by the sun is deeper than the shadow (Shevuot 6b).

3וטמא אתו HE SHALL PRONOUNCE HIM UNCLEAN — He (the priest) shall say to him. “Thou art unclean” — for it is an enactment of Scripture that hair that has become white is a symptom of uncleanness.