TT #105 · Parshas Tazria · Aliyah 2 · Passuk 8
English: The kohen shall look [at it]. And, behold! the mispachath has spread on the skin. The kohen shall pronounce him unclean. It is tzara'ath.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:8
Torah Temimah cites Sifra (Toras Kohanim) for the qualifying appearances required for spreading to render a person impure.
Sifra — Vayikra 13, Midrash Sefer
English: A spreading lesion renders a person impure only if the new area has one of the four qualifying appearances, and only in an appearance that could itself render the person impure as the original lesion.
Textual derivation
Spread establishes impurity only when it occurs in one of the qualifying appearances; a change outside the prescribed appearance categories does not suffice by itself.