Vinyl dye might work, but as lonepigeon said, ink or standard dyes probably wont work. Your best bet would be latex based paint brushed on in several thin layers. I painted a vinyl facehugger with Grenadier paints (made by same factory that does the Games Workshop paints), and after 15 years of flexing the tail and fingers, the paint is still sticking and flexing with the vinyl.
I may have a spare bottle or two of the black Grenadier paint if you wanna give it a shot, but any latex based acrylics should work.
I may have a spare bottle or two of the black Grenadier paint if you wanna give it a shot, but any latex based acrylics should work.