No offense to everyone advising this guy not to paint his helmet; respect for trying to keep him safe, but honestly... Im not recommending you do it at all, but I would argue the damaging effect of paint on the plastic shell is minimal. of course someone like an instructor or helmet company would say not to alter it in any way, such people need to be overly cautious, and with good reason.
As far as helmets being made out of the same material: not true at all, please know what you are talking about (do some research if you are not sure) before making such bold claims. I personally have a fibreglass shelled helmet (i.e. not plastic and non-reactive with spray paint). I have a feeling the Bern helmet in question here is also fibre glass (check it out though). If you can, without damaging the foam at all, remove the shell, I would do that. This will ensure no damage to the most important part of the helmet: the foam (for helmets like the sandbox one, with closed cell foam, Im not so sure. I just wouldn't use one of those at all). Also, you could get an airbrush or pneumatic spray gun and do it. I can say with a high level of certainty that this would not damage even a plastic shell of any kind (don't quote me on this though; Im just giving points to consider and would recommend you extend your research to some more reliable sources than NS before taking any action).
Hope this helps. And hey, perhaps we should direct the brain injury suggestions towards the thousands of people who just plain don't wear helmets... or mouth guards for that matter. how many people here wear a mouth guard? I know mine has saved my teeth and brain. I'm pretty sure they do an amazing job at reducing brain injury. anyways, this is getting way to long. this is a dangerous sport, have fun, be safe, but also don't take it to the point where you're just loading yourself up with padding so you can 'progress'. it will forever be dangerous as that is the nature of skiing. (i feel that this is the direction mountain biking has taken, and for anyone who was at Crankworks I think you may have an idea of what im talking about: lots of cool stuff, but a lot of stupid, huge, dangerous stuff too). end of rant. i have an assignment to do.