Putting aside the stuff about the paint ruining the plastic shell, which must be irrelevant because the plastic shell is just cosmetic and it’s the chunk of styrofoam that separates your noggin from the road – has anybody ever changed the colour of their helmet with a rattle can? Or taken a plain helmet and sprayed a flash of a different colour onto part of the shell?

NOOO! You’ll melt the expanded polystyrene and die a horrible death. Probably

Even if you mask off the styrofoam?

You’ll get loads of paint in your hair (unless you mask off your head as well as the Styrofoam)

Yep, done a few. Masking all the polystyrene takes ages. I mean ages. Don’t forget to mask the holes for the peak bolts as well.
Make sure you scuff up the surface to key in the paint, I use a not scratch dish wash pad thing, and you need to key it in between each coat of paint, and before you lacquer it.
Ps, you’ll probably die.