Although it sounds like a good idea and I bet lots of people might want one it is not safe to do so.
A helmet is made up of resins and others stuff and when other chemicals like paint are put on the helmet it can weaken the structure of the helmet.
You may want to get some of those non-DOT helmets that some people wear and do it on those since it will not void the warranty or DOT rating.
Get about 10 and do some flames or some skulls and stuff like that and set up at a bike show and see what happens. It will take a season or two to get established but once you do establish yourself you will get a following and you can make some serious bucks. Next people will bring you their gas tanks and bike fenders and stuff like that.
Thank you for the tip, I wonder if there would be a catalyst that you can put over the paint that would maintain the safety specifications of the helmet. I wouldn't want anyone to be hurt because they had a cool design on their helmet. You have given me some research to do, thanks again.