Would sticking stickers on a helmet or painting them etc make a helmet less useful in a crash? Not knowing the science of helmets but perhaps stickers in the wrong place might make it not behave how the manufacturer intended during a crash (not breaking apart correctly)?
I agree with the others, it doesn't seem like stickers should be a problem. My helmet has stickers on the shell added by the manufacturer - clearly they were added make me ride faster.
Paint is a different story... Polystyrene (aka Styrofoam) and certain other plastics can be damaged by the solvents used in paints, like some spray paints. I believe petroleum solvents in particular can cause polystyrene to melt.
Polystyrene is sometimes used in crafts and model building, so a hobby store might stock some polystyrene safe paints or be able to recommend a brand of paint that'll be safe to use.
As long as the material in the paint doesn't cause the helmet material to degrade, it should be fine.
While helmets do sometimes break apart in a crash, that's not a primary design requirement for them to keep your head protected. The two things a helmet does to keep your head safe during a crash are compress (to absorb the impact) and stay securely fastened to your head (so the foam stays between your head and the hard thing the helmet is hitting).
Breaking apart is very much a secondary thing that simply sometimes happens after the primary purpose (compressing) happens to destroy the bits holding the helmet together.
If the stickers are a hard raised item that will compromise the effectiveness of the helmet, however, since the protrusion could be driven through the foam in a crash.
Stickers are fine... paint is the danger as mentioned already.
Some helmets are constructed using a thin plastic shell that is stuck to the foam base. If you remove the shell and paint that seperately, then re-attach it you should be fine.
However, as you should get a new helemt every few years, to make the manufactures happy and to be safe because the materials break down over time due to UV light and other variables, I'd just go out and find a nice helmet in the colour that you want. The cost isn't extremely high and you'd look very chic ;-)
The Bicycle Helmet Safety Institute writes in an article:
The Snell Memorial Foundation has informed us that the hundreds of crashed helmets they examined in a study done in the mid-1990's with Harborview Injury Prevention Center showed no ill effects from any of the stickers that had been applied.
But duct tape can compromise a helmet. Details are in the article.