Mikvaot, Chapter Seven, Mishnah Five
1) If a kortov of wine fell into three logs of water and its color became like that of wine, and the water then fell into a mikveh, it does not render it invalid.
2) If there were three logs of water less a kortov into which a kortov of milk fell, and their color remained like the color of water, and then they fell into a mikveh, they do not render it invalid.
a) Rabbi Yohanan ben Nuri says: everything goes by the color.
Section one: Wine doesn’t disqualify a mikveh. Drawn water does. So what do we do with a mixture of the two? According to the mishnah, if the color is like wine, then the water is treated like wine.
It is interesting to note that since in the time of the Mishnah people usually drank their wine mixed with water, a cup of wine/water would probably be in their eyes, wine, not water mixed with wine. In other words, if it has the color of wine, its wine.
Section two: In this case, a little bit of milk, which does not disqualify the mikveh, falls into almost three logs of drawn water, which are not quite sufficient to disqualify the mikveh. There are now three logs of what looks like drawn water, but since there are not actually three logs of drawn water, the mikveh is not disqualified.
Rabbi Yohanan ben Nuri disagrees with this case. Since the color is that of water, the substance must be treated as water which does disqualify the mikveh. This would make the rule in section two consistent with that in section one.