Michael,
theory says the signal will be degraded much worse than it really will, especially if the "wrong wire" part of the run is short. If the whole wire is not 75 ohms impedance, then the energy will not all go just from one end to another, some of it will bounce back and forth on the wire, which might show up as a smear in the picture (might not). Again, how long is the run? You can probably get a good picture with twenty feet of lamp cord instead of a video cable if there is no source of interference nearby, but this is not a reliable approach, so we never bother to mention it. And because we don't mention it, people get all sorts of uptight ideas about what can be done...I once heard that you cannot put a camera signal on RG-6! just because the installers were used to using RG-59. It could be as simple and silly as they had no connectors for RG-6, so you could not put a camera signal onto it.

THEman,
you are right, the impedance will change, but it will take a heck of a lot more sophisticated equipment than a multimeter to check it at all.