You cant just pull out a can of paint and spray a helmet!!!!
You have to get it done by a pro who will pull it appart and paint it to the correct standards.
If you use a pro helmet painter they mostly only work with new white helmets and charge a lot more for a old or pre painted one because of all the prep work required. The helmet will be able to be used for competition CAMS because the painter will have the paper work for the helmet.....
It will cost you $500 to $800 for a shmick paint job that is original an looks awsome plus the cost of a new motorsport helmet $500 to $1500.
If you are a tight ass and do it on the cheap you put your self at risk
(if you have a $2 head then wear a $2 helmet) most people are happy to spend $1500 on car mods but balk at that for a helmet???????
I have raced Karts, Rally & Motocross and have had my share of crashes, so dont skimp on helmets....
P.S if you have a smash in your helmet or drop it on the road or a hard surface then throw it away because they only work once.
check out
http://www.troyleedesigns.com/paint/index.html