Commentary for Leviticus 13:6

כֵּהֶה means it has become paler than its former color — consequently if it remains in its color or if it has spread he is unclean.

AND THE PRIEST SHALL SEE HIM A SECOND TIME THE SEVENTH DAY, AND BEHOLD, IF THE PLAGUE BE ‘KEIHAH’ (DIM), AND THE PLAGUE BE NOT SPREAD IN THE SKIN, THE PRIEST SHALL PRONOUNCE HIM CLEAN. Rashi commented: “Keihah means it has become paler⁹¹Since the term nega (plague) is masculine, and keihah (pale) is in the feminine, the word keihah cannot be an adjective to nega [meaning: “if the plague is pale”]. Instead, it must mean, as Rashi points out, “if the plague has become pale,” serving as a verb. than its [former] color. [This allows the inference] that if the plague remains in its former color and has not extended in the skin, he is impure.”
This indeed is the sense of the verse.⁹²For since the verse states two conditions for the priest’s pronouncement that the person is pure — namely, if the plague has become paler, and the plague be not spread in the skin — it is obvious that if only the first condition has been met, but not the second, that the person is impure. This is the meaning of Ramban’s comment on Rashi’s statement. But the interpretation of our Rabbis is not so, for we have been taught in the Mishnah:⁹³Negaim 1:3. “To cause to be put in quarantine [for a second week] such a plague which continues unchanged at the end of the first week; to pronounce pure such a plague which continues unchanged by the end of the second week.” And in the Torath Kohanim the Rabbis have expressly said⁹⁴Torath Kohanim, Negaim 2:8. that in the case of garments, if the plague is at a stay at the end of the first week, they are to be put in quarantine [for a second week], and if it be at a stay at the end of the second week, they are to be burnt; but in the case of a person, if the plague be at a stay at the end of the first week the priest is to put him in quarantine for another week, but if at the end of the second week it is still at a stay he is to pronounce him pure. And in Tractate Megillah the Rabbis have further said:⁹⁵Megillah 8 b. “This excludes a leper who has been put in quarantine for a week, whose state of leprosy is determined not by his bodily condition, but merely by days.”⁹⁶“For if on the seventh day the symptoms of impurity are not found in the plague, namely, the black hair in it has not turned white, or that the plague did not spread in the skin, the priest will pronounce him pure, even though the plague is still at a stay” (Rashi, ibid.). Now if it were necessary that the plague should become dim, then his purity would be dependent upon his bodily condition! Rashi himself explained it there in such language, saying that the leper’s purity is not dependent on the physical state of the plague, for if at the end of seven days a symptom of impurity — white hair or extension of the plague — is not found, the priest is to pronounce him pure although the plague has stayed in its appearance, that is at the end of the second week.⁹⁷If the plague has stayed in its appearance at the end of the first week that the infected person was shut up, Verse 5 clearly states, then the priest shall shut him up seven days more. The discussion here relates only to the end of the second week.
Rather, this is what the Sages said:⁹⁸Torath Kohanim, Negaim 2:6. at the end of the second week, whether the plague has paled from the color of snow to [the shade of white of] the lime used in the Sanctuary, or like the white of an egg’s membrane, or even if it has become stronger, namely, that it was at first like the lime [used in the Sanctuary] and then [at the end of the second week] it had become bright-white like snow, and all the more so if it remained in its original color — as long as it did not spread in the skin, the priest pronounces it pure. If so, the interpretation of this verse is as follows: “if the plague be ‘keihah,’ meaning that it has turned into the color of another plague, such as from that of snow to that of lime, since it has not spread in the skin, the priest shall pronounce him clean; it is but a scab.” For in order that one should not say, “since the plague has changed into the color of another plague it must be inspected from anew,” Scripture therefore expressly taught that he is deemed pure. The same law applies if the color became stronger, since Scripture has already taught you that a change from color to color is not a symptom of impurity, but rather is considered as if it is at a stay, and as long as it did not spread in the skin [the afflicted person] is pure. Should you ask: “But why did Scripture not mention expressly the case of a change to a stronger color [as being pure], and we would know that this is all the more so if it became paler?” [The answer is that Scripture] came to teach you that although it became paler, if it spread in the skin he is nevertheless impure. Now the meaning of the word keihah is that the plague has become paler, changing to one of the colors of leprosy-signs, such as from that of snow to that of an egg’s membrane, which can still be a leprosy-sign. But if it has become paler than the colors of leprosy-signs, in that case the person is already healed, and there is no longer a plague; thus even a spreading thereof no longer renders him impure at all. In a similar manner to this presentation has it been explained in the Torath Kohanim.⁹⁸Torath Kohanim, Negaim 2:6.

והנה כהה הנגע, and the affliction has dimmed, etc. Rashi explains this to mean that if the appearance of the affliction remains either stationary or has spread, the person suffering from it is impure. Maimonides writes in chapter one of his treatise Hilchot Tum-at Tzora-at that the word כהה means that if the appearance is less white than any of the four degrees of whiteness the Torah had described, the person afflicted by it is now טהור, "clean." Similarly, if it neither spread nor dimmed nor sprouted a white hair in the area of the affliction, the person who suffered these symptoms is "clean" also. Thus far Maimonides.