a couple questions before you get started. first off, what kind of helmet do you have? cold cast or fiberglass resin?
if it's cold cast then you should sand it down real good, to about 800 grit before you tape it all off. then use rustoleum automobile primer (it's the best!), sand that lightly with 800 grit, then get Krylon Ocean Blue (available at ACE hardware in a small can) for the light blue, wait overnight, tape off the mandibles and head strap then rustoleum royal blue for the ears and cheeks.
if it's a resin helmet then you're going to want to sand it down anyway, all the way to 1000 grit. again use the rustoleum automobile primer and then lightly sand that with 800 and 1000. then get yourself some silver leaf rub'n'buff. let it set for a few days after you get it rnb'd real good then go through just like a cold cast with the two blues i mentioned already.