TT #83 · 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 how the roots and tips of two hairs determine the sign.
Sifra — Vayikra 13, Midrash Sefer
English: “Hair in the affection has turned white.” The Mishnah teaches: if two hairs are black at their roots and white at their tips, the person is pure; if the roots are white and the tips black, the person is impure.
Temporal distinction
The status depends on the color at the roots of the hairs: white roots establish the sign, while black roots do not.