…buy a tin of hammerite. It wont damage the plastic…
Not sure if serious…. :-/
The outer shell will probably be polycarbonate, and it is there to hold the brittle polystyrene foam together while it does its stuff in a crash. Polycarbonate is attacked by loads of solvents. Even if it doesn’t dissove straight away, the damage can take the form of embrittlement and/or stress cracking. Stress cracking can take some time to appear. In the case of embrittlement, you wont see anything until you hit the helmet with something (the ground?), and the shell cracks – Even the solvent in the glue on some stickers can affect polycarbonate.
So you *really* need to know what you’re doing if you’re going to paint anything made of polycarbonate, let alone something that’s designed to stop head injuries… (Fibreglass m/c helmets can be painted without problems, though.)
High-Vis black?