Midrash su Levitico 13:31

1) (Vayikra 13:31) ("And if the Cohein sees the nega of the nethek, and, behold, its appearance is not deeper than the skin, and there is no black hair in it, then the Cohein shall quarantine the nega of the nethek for seven days.") "And if the Cohein sees the nega of the nethek": R. Shimon says: What is the intent of this? Nethek is being likened to nega. Just as a nega confers tumah only if it precedes the white hair, so a nethek confers tumah only if it precedes the yellow hair.

2) (Now why is a verse needed for this?) Does it not follow a fortiori? viz.: If white hair, which other hair does not "rescue" (from conferring tumah [i.e., black hair in that site does not prevent the white hair from conferring tumah]) — (If white hair) confers tumah only if the nega precedes it, how much more so should thin yellow hair, which other hair does rescue (from conferring tumah [i.e., black hair in that site does prevent the yellow hair from conferring tumah) — (How much more so should yellow hair) confer tumah only if the nega precedes it! — No, (the verse is needed, for) this may be the case with white hair which is powerless in its place to confer tumah within (a nega of) any appearance, (but only in a nega of one of the four "appearances"), as opposed to thin yellow hair, which has the power in its place to confer tumah within (a nethek of) any appearance. And since this is the case, I would say that it confers tumah whether or not the nethek precedes it. It must, therefore, be written "the nega of the nethek." Nethek is being likened to nega. Just as a nega confers tumah only if it precedes the white hair, so a nethek confers tumah only if it precedes the yellow hair.

3) R. Yehudah says: Wherever the stipulation of precedence (of nega to hair) must be made, it is made (explicitly, in the verse itself), but nethek, about which it is written (Vayikra 13:32) "and there is no yellow hair in it" (without any explicit stipulation of precedence), (nethek) confers tumah whether or not it precedes (the yellow hair).