Midrash sur Le Lévitique 13:4

1) (Vayikra 13:4) ("And if it is a white bahereth in the skin of his flesh, and its appearance is not deeper than the skin, and its hair has not turned white, then the Cohein shall quarantine the plague-spot for seven days.") This tells me only of a white bahereth. Whence do we derive for inclusion a (white) se'eth? From (Vayikra 13:10) "a white se'eth." Whence are other (white) appearances derived? From "And if a bahereth."

2) I might think that just as it (bahereth) is third in order of the verse (Vayikra 13:2), it is third in order of appearance (i.e., of brightness); it is, therefore, written "white." It is white and there is no (white) above it in brightness. And what is its whiteness like? Like (the whiteness of) snow, as it is written (Bamidbar 12:10) "And, behold, Miriam was as leprous as snow."

3) I might think that every appearance of snow is tamei and that all other appearances are tahor. It is, therefore, written (Vayikra 13:39) "It is a bohak (brightness) … it is clean": It is clean, but everything above it (in brightness) is tamei.