Or HaChaim on Leviticus 13:19/אור החיים על ויקרא י״ג:י״ט

1 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.

2The 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.