Had the exact same helemt, and now its black with purple cirlces. Just go to walmart and buy some plastic adhesive krylon (comes in shades of white and black) then what ever colors you want. Tape it all over because the paint will disolve the foam. To prep it just sand it with a suuppper fine grit sand paper.
Literally even a scratch the size of a hair before you paint it will be noticable after (just somehting to think about). And just cause the can says it takes 15 minutes to dry doesnt mean you should wait just 15 minutes. It was super easy to do, just took a while.
And finish it with some clear coat gloss (walmart) but make sure it is not going to melt the paint. you can test this on a milk jug or something like that. I accidently sprayed it too close to the purple and it made on of the circles get all wavy and shrivled, so I just sanded it out and used a little googone and it was fine.