Great responses so far guys, thank you. I am confused about this though:
Heffling wrote:Matt Medium kills shineyness, and doesn't really dull a color.
My thinking is that since
MM dries clear, if you mix it with paint you can get a thin and/or somewhat trsansperant red or yellow color coat, so that whatever was underneath (in my case grey) would partially shine through. But since no one recommended this option seems like a moot point. Now I just have to decide between the grey or brown/purple mixing routes....