TT #84 · Parshas Tazria · Aliyah 1 · Passuk 3
English: The priest shall examine the affection on the skin: if hair in the affected patch has turned white and the affection appears to be deeper than the skin, it is a leprous affection on the skin; and if the priest sees that it is a leprous affection, the priest shall pronounce the person impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:3
Torah Temimah cites Sifra (Toras Kohanim) for the required sequence between the appearance and the white hair.
Sifra — Vayikra 13, Midrash Sefer
English: “Has turned white” means it was not white beforehand. If the baheres preceded the white hair, the person is impure; if the white hair preceded the baheres, the person is pure.
Textual exclusion
The order of appearance matters: the baheres must precede the white hair for this sign of impurity.