Mikvaos, Chapter 7, Mishnah 5. Our chapter has been dealing with the subject of shinui mareh, a change in the appearance of the water, and this mishnah continues that theme by applying it to one of the most fundamental rules of a mikvah.
The rule we work with is this: three lugin of mayim she'uvim (drawn water) that fall into a mikvah before it has reached its full mem se'ah of rainwater render that mikvah posul. Three lugin is the critical measure. Less than that does no damage. Our mishnah asks what happens when the identity of those three lugin becomes questionable because of a change of color.
Water That Looks Like Wine
Begin with the opening case. The mishnah speaks of "sheloshah lugin mayim venofal letochan kortev yayin": a full three lugin of water are sitting there, and one kortev, the smallest droplet, of wine drops in. Wine being what it is, that single droplet is potent enough to tint the whole volume, so that the liquid no longer looks watery at all but wine colored. In the words of the mishnah, "vaharei mareihen kemarei yayin." That liquid is then poured into a mikvah still short of its mem se'ah, and the ruling is "venoflu lemikvah, lo paslu": the mikvah is not spoiled.
The reasoning is straightforward. What entered the mikvah no longer presents itself as water at all; it presents itself as wine. Since its appearance has changed to the appearance of wine, it no longer carries the halachic status of water. And the Chachomim issued their decree only concerning three lugin of mayim. This is not mayim; it is yayin.
Water Completed by Milk
Now turn the situation around. Here the mishnah presents "sheloshah lugin mayim choseir kortev": a quantity of water that falls one tiny droplet shy of three lugin. Were this poured into an unfinished mikvah as it stands, the mikvah would be unaffected, since the disqualifying measure of three lugin has not been reached. But the mishnah adds "venofal letochan kortev cholov": in drops that missing droplet, and it is milk. Volume wise the liquid is now a complete three lugin. Milk, however, is white, so the mixture retains a watery look, and the mishnah says "vaharei mareihen kemarei mayim," their appearance is that of water. Such a liquid enters the mikvah, and the ruling is "venoflu lemikvah, lo paslu."
Why not? According to the Tanna Kamma, we do not say that the drop of milk is botel in the water and therefore counts as water. Bitul can tell us that the milk is disregarded, but bitul cannot manufacture a shiur. To disqualify the mikvah we need a genuine measure of three lugin of water, and here the water itself falls short of that measure. The mikvah therefore remains valid.
The Position of Rebbi Yochanan ben Nuri
"Rebbi Yochanan ben Nuri omer: hakol holech achar hamareh." Rebbi Yochanan ben Nuri disagrees and rules that everything follows the appearance. Since the liquid looks like water, it is treated as three lugin of water in full, and if it is she'uvim it disqualifies the mikvah.
Notice how neatly the two cases frame the dispute. In the first case, appearance worked in the mikvah's favor, and all agree that liquid looking like wine is not the water of the decree. In the second case, appearance would work against the mikvah, and here the Tanna Kamma insists that appearance cannot substitute for a real measure of water, while Rebbi Yochanan ben Nuri holds consistently that appearance is the whole determination.