or there's the good old standby's. toothpaste or mustard. i prefer toothpaste because on the paint that i use the mustard makes the silver look not very silver. also, i like the smell of mint over mustard. and you can get a little more delicate with toothpaste because you don't really need to glop it on as thick to protect the layers.
but after you've got all your layers on, you can put the helmet under hot water and rub it off to reveal the layers of color. a few word of caution though: one, if you get on a roll and put down all your layers in a short period of time, wait at least 2 hours before pulling your reveal. the paint needs some time to dry or you'll pull off a lot more than just the masked areas. second, don't go too slow with the process. don't let the mask sit too long. once you start the project you should try to have it done in a week at the most, otherwise the toothpaste (or mustard) will harden up too much and be a real pain to remove after you have all the colors on the bucket.