Can I paint over RedGard waterproof membrane?
Ideally, you'd take the time to put tile there. You can cut tile fairly easily to fit. 

If not, I imagine you can. I think I overlapped into the sheetrock area of my bath with redgard when I did it and the paint is fine, but I don't completely recall how far I took the RedGard. As long as you use a good primer, you should be OK, though.