Negaim, Chapter 11, Mishnah 4. This chapter is devoted to nigei begadim, the tzaraas that appears on materials. When the Torah speaks of such tumah it has a very limited list in mind: garments woven of tzemer rochel (the wool of a lamb) or of pishtan (flax), and oros, animal hides. Our mishnah asks a question about all of these: does the color of the material matter? And note carefully what is being asked. The question is not which colors of nega bring tumah, but which materials are susceptible to receiving that tumah in the first place.
The mishnah opens: "Ha'oros vehabegadim hatzvu'im ein mitamim binega'im", hides and garments that are colored do not become tamei with nega'im. Neither a hide nor a woven garment of wool or flax is susceptible once it carries a color, and the Tanna draws no distinction whatsoever in how it came to be colored. Whether a person dyed it or whether the material simply grew that way, it stands outside the laws of nigei begadim.
Once color has become the issue, the Tanna takes a moment to state the law of houses too, although the subject of this chapter is really materials. Regarding "habatim", houses, the ruling is "bein tzvu'im", whether they are colored, "bein she'einan tzvu'im", or whether they are not colored, "mitamim binega'im", they can contract this tumah. Plain white walls and colored walls are treated identically, and once again the origin of the color changes nothing: painted by human hands or colored bidei shamayim, by nature itself, the house still falls under these halachos. "Divrei Rebbe Meir", so holds Rebbe Meir, who thereby groups oros together with begadim on one side of the line and leaves batim on the other.
Rebbe Yehudah disagrees about hides. "Rebbe Yehudah omer, ha'oros kebatim", hides follow the rule of houses rather than the rule of garments. Rebbe Meir had equated oros with begadim, ruling that once they carry any color, natural or applied, they are not susceptible. Rebbe Yehudah has pesukim from which he derives that hides belong in the category of houses, and therefore they remain susceptible to nega'im, and, as the Rav spells out, "va'afilu tzvu'im bidei adam mitamim binega'im", even if a person dyed them.
Rebbe Shimon also addresses himself only to hides, and he takes a middle position. Hides that are colored bidei shamayim, whose coloring is the natural coloring of the animal, are susceptible to tumah, while hides colored bidei adam, dyed by a person, are not.
The Rav follows Rebbe Shimon in the matter of hides, and not the Tanna Kamma. It is worth observing that only oros are under debate here. When it comes to a garment woven of wool or flax, all the Tannaim hold one view: any color it bears, whether a dyer gave it or whether it belongs to the material naturally, removes it from this tumah completely. The halachah is therefore formulated "shehabegadim ein mitamim tzvu'im", that colored garments are not subject to nega'im, "afilu bidei shamayim", even where heaven gave them their color.
Where is this learned from? The pasuk says "beveged tzemer o beveged pishtim". Flax is being spoken of in its natural state, as it comes into the world, and the juxtaposition teaches that wool too is spoken of only in its natural state. And the pasuk says "ulapishtim velatzemer": just as flax in its natural state is lavan, white, so too the wool of which the Torah speaks is white wool.
So the halachah as it emerges from this mishnah is twofold. Garments of wool or flax that bear any color, from man or from heaven, are simply outside the laws of nigei begadim. Hides are judged like Rebbe Shimon: "veha'oros hatzvu'im bidei adam ein mitamim binega'im, aval bidei shamayim mitamim binega'im". Dyed by a person, a hide is not susceptible; colored naturally, it is.