TT #109 · Parshas Tazria · Aliyah 2 · Passuk 10
English: The kohen shall look [at it]. And, behold! there is a white se'eith on the skin, and either it has turned the hair white, or there is healthy, live flesh in the se'eith.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:10
Torah Temimah cites Sifra (Toras Kohanim) for the requirement that the relevant change occur throughout the affected area, not only in part.
Sifra — Vayikra 13, Midrash Sefer
English: It turned in its entirety, not merely in part.
Textual derivation
The relevant color change must affect the whole area described by the derashah, not just a portion of it.