This is a fairly long-handled question, but it boils down to a desire to understand R' Shimon's qal vechomer in the Mishna, Negaim 10:2. Usually, for a qal vechomer to work, there needs to be a relationship of some description between its different components, and an indication that one half of the equation is more severe than the other. The following is the background to his qal vechomer and the qal vechomer itself:
In the Mishna, Negaim 10:2, R' Shimon produces a qal vechomer in order to make a point: that when it comes to tzara'at in the beard or on the head (נתק), thin, yellow hairs are an indicator of impurity only if the blemish predates it. His proof relies on the fact that in regular skin blemishes (בהרות), white hairs are only an indicator of impurity if they are preceded by the blemish. The technical term for this is הפוך: that the blemish "turned" the hair white.
In producing a qal vechomer, R' Shimon relies on a key difference between the baheret (the skin blemish) and the neteq (the blemish in the beard/head): when it comes to a skin blemish, while white hairs are a sign of impurity, there is no type of hair that can serve as a sign of purity - in other words, the presence of two white hairs is enough to render the skin blemish impure, regardless of how many black hairs there are. When it comes to a blemish on the beard or head, the presence of black or red hairs will purify the blemish, regardless of how many thin, yellow hairs there are.
His qal vechomer is as follows: Just as the white hair (in a skin blemish), which cannot be redeemed by any other hair (ie: cannot be negated by the presence of a black hair and made clean) is only impure if it was turned white by the blemish, then how much moreso should a thin, yellow hair (in a head/beard blemish), which is redeemed by other hair (ie: is negated by the presence of black or red hairs and made clean), only be considered impure if it was turned thin and yellow by the blemish!
In his lashon, "מה אם שער לבן שאין שער אחר מציל מידו אינו מטמא אלא הפוך, שער צהֹב דק ששער אחר מציל מידו אינו דין שלא יטמא אלא הפוך".
I do not understand what could possibly be the relationship between a hair's being changed by the blemish rather than predating it, and its being "saved", so to speak, by other hairs (ie: negated by their presence and made clean). Furthermore, I do not understand why one of these cases should necessarily be more stringent than the other. Is the skin blemish more stringent because no amount of black hair can purify it? Or is the head/beard blemish more stringent because black and red hair can purify it?
If somebody understands this qal vechomer and how it serves as a proof, I would appreciate an explanation. (I would also appreciate any reference to mefarshim who either explain it or find it problematic.)