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.
bicycles.stackexchange.com/users/3021/Jason Braucht
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.
bicycles.stackexchange.com/users/76/freiheit
One of the concerns with stickers and paint to a lesser degree is that it may mask or hide damage to the shell.
Manufacturers reccomend inspection prior to every use and damage prior to or after application of a sticker could be hidden.
bicycles.stackexchange.com/users/3083/mikes
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 ;-)
bicycles.stackexchange.com/users/3291/Chef Flambe
About stickers: 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.
bicycles.stackexchange.com/users/3674/unforgettableidSupportsMonica
I personally wouldn't worry about most stickers and decals.
Paint can be more of a problem, or if you used the wrong clear coating over a sticker/decal.
It's a question of whether the particular paint reacts with the plastic shell.
bicycles.stackexchange.com/users/1584/Daniel R Hicks
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)?
Do stickers or paint compromise a helmet?
bicycles.stackexchange.com
<safety><helmets><modification>
bicycles.stackexchange.com/questions/7687
bicycles.stackexchange.com/users/3258/davidsleeps
Sticker do not damage a helmet, parts of the helmet may however be damaged by the solvents found in paint. It may, nevertheless, be possible to find a suitable paint in a hobby store. Stickers do pose a potential problem in that they may hide any helmet damage, so the helmet should be inspected before the sticker is applied. Stickers have not been found to compromise a helmet's effectiveness, but the chemicals in some paints can damage the body and sometimes the shell of a helmet. Stickers could be a problem if they protrude or are used to cover existing damage, and duct tape can compromise helmets. You could use paint that is designed not to react with the helmet materials or paint the shell separately from the more sensitive polystyrene, but a better solution could be choosing your preferred colour on your next regular replacement.
Stickers have not been found to compromise a helmet's effectiveness, but the chemicals in some paints can damage the body and sometimes the shell of a helmet. Stickers could be a problem if they protrude or are used to cover existing damage, and duct tape can compromise helmets. You could use paint that is designed not to react with the helmet materials or remove and paint the shell before reattaching to the more sensitive polystyrene, but a better solution could be finding your preferred colour on your next regular replacement.