Commentaire sur Le Lévitique 13:19

או בהרת לבנה אדמדמת in OR A BRIGHT SPOT REDDISH-WHITE — It means that the plague is not evenly white but it is compounded and intermingled with two colours — white and red.

והיה במקום השחין שאת לבנה, this spot is not being judged by the same criteria as listed earlier which were inflammations of skin on the flesh; similarly, inflammations caused by fire burning the skin. When ordinary שחין destroys the tissue on top of the skin it never grows back to look as it did originally. What grows on the afflicted area is tissue which resembles the original skin, so that no skin graft is needed. Original skin is medically irreplaceable. [opinions in those days.] When someone suffers from a נגע צרעת the skin, when healed, looks indistinguishable from the original.

שאת לבנה או בהרת לבנה, "a white rising or a very bright spot, etc." According to Torat Kohanim the reason the Torah writes לבנה, white, is to exclude אדמדמת, a שאת of a reddish appearance; the words בהרת לבנה אדמדמת refer to a בהרת mixed with a reddish appearance. [I did not find this in my edition of Torat Kohanim on this verse. Ed.] How do I know that the rules stated in our verse apply to a שאת אדמדמת, and to בהרת לבנה that instead of the שחין there is now a completely white spot of the intensity called בהרת? This is why the Torah added the words נגע צרעת, "it is a form of leprosy," in verse 20. At first glance it seems that these extra words should have been necessary only in respect of שאת, the rising on the skin which was of a mixed colouring (white and reddish); seeing that the basic colour was not an intense white I might have presumed that it did not qualify anymore as a symptom conferring ritual impurity. We never would have assumed this of an area which contained the kind of bright whiteness known as בהרת. We never find that שאת is more susceptible to impurity than בהרת.