if you use a high metallic silver base then sand with a 400 grit paper and then repaint and then lightly sand with 400 grit again (be sure to be mindful of sanding stroke/direction you can create a awesome realistic metal finish. Take a sheet/strip of sintra or anything close to the surface texture you are working with and practice on it first. the metallic when sanded actually turns into a metal base
Rustoleum Royal Blue and Krylon Short Cuts Ocean Blue. If you have to get a plain helmet use Rub n Buff but I highly recommend you get a cold cast helmet, the look is much better if you get the real thing. R+B is just for touch ups then.
Also for most paint information look through the WIP threads and you will find paint lists. I try to always put paint lists in mine threads.