I’m not here to tell you what to do but most regular consumer grade helmets I’ve seen would be hard to spray the outside without hitting the foam especially near the vents… Foam which, once again, melts INSTANTLY when spray painted. Foam which is also basically the whole protective value of the helmet.

Anyways I’m sure there are helmets where it’s possible to spray paint without damaging the foam. Or you could do a good job taping. Or you could shrug at the hopefully minor foam damage. Many helmets never have to protect your head so you might be lucky there too. All I’m saying is that there are good reasons to say “don’t spray paint helmets” as general good advice.