Some kinds of plastics are extremely resistant to paint no matter what you do. These are usually toughened waxy plastics/superhard plastics like polyoxymethylene that make up parts of a figure that could easily be stressed if made from the usual pvc or abs. The limbs of the Transformers Siege Battle Masters are made of this stuff and you need to color them, so what do you do? It's RIT DyeMore for synthetics to the rescue! Mix half the bottle of dye with a cup or so of water, boil the liquid on med to high heat just so you get a gentle rolling boil and drop your parts in. I like to suspend mine in a strainer so I can 'lift and look' to check the color. In under a minute the pieces will start to be dyed with a color that won't rub off! The longer you soak them the deeper the color. Here I've dyed the lower half of Slitherfang and used testors/tamiya lacquers to paint his upper half! You can even use him with your GI Joe Classified figures as a shield or drone.
For more pics slither on over to his page here: Custom dyed Slitherfang