This is just an idea that I'm gonna try soon and I've never tried it before, which I suggest to practice with a piece of plastic or something else rather than the helmet (like a plastic bottle or something similar to the Helmet's material to avoid a failure..cause we're talking about GLUE here and not just paint!
Try adding instead of paint, using pieces of Aluminum Foil, sticked with glue, could be plastic cement or Super Glue. Apply the glue to the areas that you really want, and then stick the little pieces or scrap of Aluminum foil. Probably you want to do this by one piece-one step at the time, before the glue dries up. Once the glue is dry, apply then to the areas that you don't want to be painted with Painters Masking tape (the one with the low stick power), start painting the helmet and then remove the tape really carefully, not to pull pieces of aluminum with it. This sounds like a great idea, which I'm gonna try sometime soon. If somebody was successful, let me know.