Mishnà su Levitico 13:37

[Black hair] that sprouts up affords protection against yellow hair and against a spreading, whether it was clustered together or dispersed, whether it was encompassed or unencompassed. And that which is left [over from before the scall] affords protection against yellow hair and against a spreading, whether it is clustered together or dispersed, and also when encompassed, but it affords no protection where it is at the side unless it is distant from the standing hair by the place of two hairs. If one hair was yellow and the other black, or if one was yellow and the other white, they afford no protection.

One who has a scall with yellow hair within it is unclean. If subsequently black hair grew in it, he is clean; even if the black hair disappeared again he remains clean. Rabbi Shimon ben Judah says in the name of Rabbi Shimon: any scall that has once been pronounced clean can never again be unclean. Rabbi Shimon says: any yellow hair that has once been pronounced clean can never again be unclean.