TT #294 · Parshas Tazria · Aliyah 7 · Passuk 56
English: If the priest examines it and the lesion has faded after it was washed, he shall tear it out of the garment, leather, warp or woof.
Torah Temimah cites Sifra (Toras Kohanim) for the specified shade comparison for a lesion that has become dim.
Sifra — Vayikra 13, Midrash Sefer
English: “The lesion has become dim”—the Sifra defines the relevant dimming as reaching the second shade; a change to the third shade does not satisfy this rule.
Textual derivation
The dimming relevant to this rule is defined by the prescribed shade comparison.