It's really hard to make a paint effect on a helmet that will not fade or scrape down and will look professional and cool. In order to do a really really good job, the paint has to be either burnt in a high temprature oven or covered in some pint of protective layer.

A way more simple way to redecorate a helmet is using duct tape. like those:

http://www.tapebrothers.com/Special-Duct-Tapes-s/19.htm

We got the black and white checkers  tape a few months ago and decorated our helmets with they, it still looks as great as the day we put it on. 

Also for safety while riding at night: glow in the dark tape!

http://www.tapebrothers.com/Glow-Tape-s/253.htm