Mikvaos, Chapter 7, Mishnah 3. Our mishnah deals with shinui mareh, the halachah of water in a mikveh whose color has changed. Everything here hinges on a matter of definition. Once water has been tinted, is it still to be called water, or has it become some other liquid entirely, leaving the mikveh unfit for immersion? The Mishnah draws the line between a coloring that leaves the water essentially itself and a coloring that means the water is no longer water.
The Mishnah begins with a case that is kosher: "hediach salei zeisim va'anavim veshanu es mar'av, kasher." Picture a mikveh that already contains its forty se'ah. A person had baskets in which olives or grapes had been sitting, and residue from that fruit clung to the baskets. He rinsed the baskets in the mikveh, and as a result the water took on a greenish or purplish cast. The mikveh remains kosher. Even though the water is no longer crystal clear, nothing of the substance of the fruit itself entered the water. All that happened is a slight tinting from the residue left behind by the olives or the grapes.
Rabbi Yosi then teaches something even more striking: "mei hatzeva poslin oso bishloshah lugin, ve'einan poslin oso bishinui mareh." A person soaks the plants from which dyes are produced, whatever those plants may be, and the water in which they soaked is now colored. Rabbi Yosi rules that this colored water is still classified as water. The practical consequence cuts in two directions.
One direction concerns drawn water. Precisely because dye water has not lost its identity as water, it carries the full stringency of sheuvim. Should three lugin of such drawn dye water fall into a pit that has not yet accumulated its forty se'ah, the pit is ruined just as it would be by three lugin of any drawn water. The Mishnah's own term is instructive: it speaks of mei tzeva, water belonging to the dye, rather than of dye as a substance in its own right. Its identity as water is intact, and that is exactly why it has the power to render a mikveh pasul on account of being drawn.
On the other hand, and for the same reason, they do not disqualify through discoloration. If the mikveh already holds forty se'ah and these dye waters fall in, the mikveh stays valid even though its water is now colored. Colored water is still water.
Now the Mishnah turns to liquids that are not water at all: "naflu lesocho yayin umohel veshinu es mar'av, pasul." Wine, mohel, which is the liquid pressed from olives, or any other fruit juice, the essence squeezed out of the fruit itself, fell into the mikveh and changed its color. Here the mikveh is invalidated. This can arise in either of two situations: the pit had not yet reached forty se'ah and now its water is discolored, or it already held forty se'ah and its water became discolored. In either case the water loses its status as mikveh water, though only temporarily, until the color reverts to the color of water.
"Keitzad ya'aseh?" What is a person to do? The Mishnah answers according to the two situations. If he does not yet have forty se'ah, and wine or mohel or another fruit juice fell in, then "yamtin lo ad sheyeirdu geshamim veyachzeru marehen lemarei mayim": let him wait until rain falls. The fresh rainwater flowing in will dilute the tint and restore the natural color of water, and then the mikveh is fine. This shows us that shinui mareh is not a permanent disqualification. It is a disqualification only for as long as the water is discolored; once the color of water returns, one may immerse.
And if the mikveh already contained forty se'ah when the wine or oil fell in and discolored it, he has an additional option: "memaleh bakatef venosein lesocho ad sheyachzeru marehen lemarei mayim." He may take even ordinary drawn water, carry it on his shoulder, and pour it into the mikveh until the color of the water returns. Since the mikveh was already complete with its forty se'ah, drawn water no longer harms it, and it serves here simply to wash out the tint.
The thread running through the whole mishnah is a single distinction: water that has merely been tinted, whether by fruit residue clinging to baskets or by the waters in which dye plants soaked, is still water and still a mikveh. But the essence of the fruit itself, wine and oil and juice, once it changes the appearance of the mikveh, has turned the water into something else, and the mikveh waits until its own color returns.