TT #106 · Parshas Tazria · Aliyah 2 · Passuk 9
English: If a man has a lesion of tzara'ath, he shall be brought to the kohen.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:9
Torah Temimah cites Sifra (Toras Kohanim) for the rule that a patch of healthy flesh can render impure a person whose skin is otherwise entirely white.
Sifra — Vayikra 13, Midrash Sefer
English: This includes one whose whole body has become white, so that a patch of healthy flesh renders him unclean.
Textual derivation
When the skin is otherwise entirely white, the appearance of a patch of healthy flesh (מחיה) is the sign that renders the person impure.