Kommentar zu Wajikra 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.

פתוך. In Shavuos (6a) Rashi explains פתוך means intermingled.

או בהרת לבנה אדמדמת, “or a white rising or a bright spot, reddish white;” according to Torat Kohanim, the reddish looking spot is a phenomenon that occurs with all the four different types of the skin diseases described in this chapter. It is a clear sign that the afflicted person will be declared ritually impure.

The true explanation of the verse then is this: "a white rising on the skin of the kind called שאת לבנה or an intensely white spot called בהרת which contains also some reddish appearance is still a symptom of ritual impurity because the colour of בהרת contains some rising, שאת, and will not result in purification seeing a leprosy like appearance grew in it." The words נגע צרעת are not necessary except to tell us that the שאת underwent a dimming of its white colour. While it is true that the author of Torat Kohanim speaks of two divisions, the second one is mentioned as incidental not because we needed the words נגע צרעת to convey this information.