Negaim, Chapter 1, Mishnah 3. The opening chapter of the masechta is devoted to the mar'os negaim, the shades of white that a nega on the skin can take, from the brilliant white of snow down through the dimmer whites. Our mishnah adds a further set of colorations: what happens when one of those whites is not pure white at all, but has some red blended into it.
The Tanna Kamma here is Rebbe Yishmael, and his term for such a blend is pasoch, a mixture. He teaches: "Hapasoch shebasheleg keyayin hamazug basheleg": when red is mixed into the brightest of the whites, the snow-white, the resulting shade is like wine that has been mixed into snow. "Hapasoch shebasid k'dam hamazug bechalav": the mixture found in the white of sid, of lime or whitewash, looks like blood mixed into milk. "Divrei Rebbe Yishmael."
The Tosefos Yom Tov notices an oddity in the way these two comparisons are built. For the shade of snow, the mishnah illustrates the mixture by wine poured into snow itself, staying with the very same substance. But for the shade of sid, it does not speak of red mixed into lime; it switches to blood mixed into milk. Why the change? He explains that it is a matter of how these substances behave. Snow is soft and porous, and wine blends into it readily, so snow itself can serve as the model. Lime and whitewash, however, are hard, and red will not blend into them in a way that produces a visible, even shade. To convey the coloration of a nega that is the white of sid tinged with red, the mishnah therefore reaches for a different white substance, milk, into which blood does mix, and there one sees exactly the appearance being described.
The second opinion in the mishnah belongs to Rebbe Akiva, who abandons the word pasoch altogether. He describes the reddish tint with the term adamdam, meaning a vivid red, and he applies it "shebazeh veshebazeh," to this white and to that white alike. The word adamdam is taken directly from the pesukim of the parsha. Why would two Tannaim choose different vocabulary for one and the same phenomenon? The Meleches Shlomo answers that although the Torah treats negaim at length in parshas Tazria-Metzora, the great majority of the halachos in this masechta came down as Torah shebe'al peh, handed from rebbi to talmid. Since a person is obligated to pass on his learning in the exact wording he received from his teachers, we may infer that the term used by Rebbe Yishmael's rabbeim was pasoch, while Rebbe Akiva's rabbeim taught the word adamdam.
Their disagreement runs deeper than terminology. In Rebbe Akiva's view, the reddish tint in each of the two whites, that of snow and that of sid, resembles wine blended with water: "keyayin hamazug bamayim." The two are nevertheless not identical, for they differ in strength. In a nega of snow-white the red emerges as "azah," forceful and striking, since the white beneath it is the most brilliant of all. In a nega whose white is that of sid, that very same wine-and-water tone presents itself as "deheh mimenu," weaker and paler than the adamdam that belongs to the snow-white shade.
So the two Tannaim give us two ways of charting the reddish mixtures: Rebbe Yishmael with two distinct comparisons, wine in snow and blood in milk, and Rebbe Akiva with a single comparison, wine in water, appearing once brightly and once faintly according to the white in which it is set.