Commentary for Mikvaot 7:4

אין בו מראה מים ארבעים סאה – if there aren’t in the Mikveh forty Seah that have the appearance/color of water, he should not ritually immerse in that Mikveh even on that side that has in it the appearance/color of water, but if he ritually immersed, his immersion has no effect.

If wine or the sap of olives fell into the mikveh and changed the color of a portion of the water, if there is not a portion [of the mikveh] that has forty seahs with the color of water, one may not immerse in there. If the wine or sap from olives changed one part of the mikveh, meaning somehow the color stayed on one side, then one cannot immerse in the mikveh unless the remaining side that still has the color of water has at least forty seahs. I'm not really sure how practical this is (the color would assumedly dissipate pretty quickly throughout the mikveh), but it does teach a principle. The colored water is not usable as a mikveh but it doesn't disqualify the mikveh. Therefore, we can effectively ignore the fact that there is colored water on one side of the mikveh such water doesn't count, but doesn't disqualify.