Just happened to spot this question, and did a little light research just now.
Evidently there's a greater range of answers than I suspected. As in, oil, acrylic, airbrush paint (which I guess is usually water or acrylic-based), etc.
The question of course is-- how
good are those answers?
This guy seems to make some strong arguments for airbrush paint, that's for sure:
I never touched up PF paint myself, so it's all still speculative to me.