TT #97 · Parshas Tazria · Aliyah 2 · Passuk 6
English: And the kohen shall see him on the seventh day a second time. And, behold! the lesion has become dimmer, and the lesion has not spread on the skin, the kohen shall pronounce him clean. It is a mispachath. He shall immerse his garments and become clean.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:6
Torah Temimah cites Sifra (Toras Kohanim) for the meaning of a lesion becoming dimmer relative to its original appearance.
Sifra — Vayikra 13, Midrash Sefer
English: One might think “dimmed” means below the four prescribed appearances; Scripture teaches that it became dimmer than its original appearance.
Textual derivation
“Dimmed” is comparative: the lesion must have become less intense than its own earlier appearance, not necessarily fallen below all four qualifying shades.