I am curious about this, as well, and surprisingly, i din't find much info on the web. I have also heard that some paints may cause damage to the shell of a helmet and some will not adhere well. I think the best way to find out what will and will not work would be to ask somebody who paints helmets/ cars/ ect for a living. It may be hard to find a person who is willing to give up the info, but who knows.
If you do learn anything, post it here, and share with the rest of us.
A side note- I called Shoei and asked them about it and the guy said they recommend having it professionally painted because only a professional would know what kind of paint to use and it "may not look good" if i do it myself. Which sort of led me to believe that the biggest risk is just not having the paint adhere correctly, or drip or something. But these are just aesthetic risks. He also said that it's completely fine to put any kind of stickers on a helmet.