The solvents in an oil or acrylic-based paint will slowly break down the structure of the helmet's outer shell. You'll need a water-based paint, which could take several coats to adhere. Furthermore, the propellant in a spray can can seep between the pores of the plastic, and once the temperature increases enough it can actually expand enough to stress the shell to the point of cracking. Make sure you use compressed air. If you can get an adequate water-based paint, go with a lighter colour, as the darker hues can attract toxic mold, which will breed inside the foam cushioning and can become FATAL.
J/K I made all that shit up.
