Hiya,
I've seen lots of places on the internet that airbursh helmets... just do a search on the net for "Airbrushed Motorcycle Helmets", here's a couple:
http://web.utk.edu/~cnewell/helmet_index.htm
http://ginag.com/helmets.html
BTW, it seems a lot of people paint their helmets... I dont know if this is True or not, but I've heard that once the helmet is originally factory painted that you can actually weaken the structural integrity of a factory helmet by having it painted... something to do with the solvents weakening the helmet material? I would also not recommend whoever paints your helmet to begin by sanding it first as oversand would also weaken the helmet.
Good Luck, post some pics when you get it painted
M.