I don't know if this thing would work but I've recently found a way to paint silicon. I haven't tried it yet though.
Basically, you need some translucent silicon for bathroom joints, oil painting and lighter fluid.
You dilute the translucent silicon with the lighter fluid and mix your paint with it. When it dries up, the lighter fluid evaporates and what remains is a pigmented silicon layer.
Smooth-on makes a product called Psycho Paint that would work well for something like this. It's a silicone based paint that is mainly used for latex masks. More info here:
I bet it would work for soft plastics. I have not personally tried it for action figures since I have only been using acrylic paints, but if it works for latex masks which are much more flexible, it should work for plastic too.