Sforno on Leviticus 14:56/ספורנו על ויקרא י״ד:נ״ו

1ולשאת ולספחת ולבהרת, even though the common denominator of these three skin eczemas is that all of them are a shade of white, and that the rules applying to them are basically similar, so much so that they can be treated as cumulative, as we know from tradition (negai-im 1,3) anyone who undertakes to rule on these matters has to be an expert in all the different shadings of white, and where these are most likely to surface. The areas in which shechin or michveh, (burnt like appearance) are judged by a combination of appearances., whereas tzarevet and the areas where hair grows are not judged by appearances at all.