Midrash su Levitico 13:37

9) "and there is no black hair in it, then he shall quarantine the nethek," but if there was (black hair) in it, he is exempt. — But perhaps the meaning is that in the absence of black hair he is to be quarantined, and in its presence, he is tamei! — This cannot be,) for (Vayikra 13:37) "And in his eyes the nethek has remained as it was, and black hair has sprouted in it … he is tahor" indicates that black hair is a sign of taharah in a nethek. How, then, am I to understand "and there is no black hair in it, etc."? (As meaning that) if there was (black hair), he is exempt. "then the Cohein shall quarantine the nega of the nethek for seven days.": This is the first quarantine.

14) (Vayikra 13:37) ("And if in his eyes the nethek has remained as it was, and black hair has sprouted in it, then the nethek has been healed; he is clean. And the Cohein shall declare him clean.") "And if in his eyes": This tells me only of his (the Cohein's) own eyes. Whence do I derive for inclusion the eyes of his son or the eyes of his disciple? From "And if in his eyes" (and not necessarily the Cohein's eyes) the nethek has remained the same. "hair": the minimum of hair — two. "black": This tells me only of black hair. Whence do I derive the same for green or red hair? From "and hair." "sprouted in it": though it not be surrounded within it. This tells me only of (hair) remaining in the beginning (viz. Vayikra 13:31) and sprouting in the end. Whence do I derive the same for sprouting in the beginning and remaining in the end? From the identity (gezeirah shavah) "and hair" (Vayikra 13:31) - "and hair" (Vayikra 13:37).

15) "the nethek has been healed": If one had a nethek with yellow hair in it, he is tamei. If black hair arose in it, he is tahor. Even if the black hair leaves it, he remains tahor. R. Shimon says: Any yellow hair which was rendered tahor for one moment never becomes tamei again. R. Shimon b. Yehudah says in the name of R. Shimon: Any nethek which was rendered tahor for one moment never becomes tamei again.