My first thought would be - DON'T.
Often, even with the new materiel used for helmet shells, paints can act as a solvent, softening the shell. Not a good thing for something that's supposed to protect your noggin. Many times, colors and graphics are molded into the resins prior to the clearcoat layer being applied.
It wouldn't hurt to contact a local custom paint shop and see what they have to say or sell (products, not services, per se). There might be workarounds as far as newer paint and pigmenting stuff, that won't compromise the integrity of the helmet.
Often, even with the new materiel used for helmet shells, paints can act as a solvent, softening the shell. Not a good thing for something that's supposed to protect your noggin. Many times, colors and graphics are molded into the resins prior to the clearcoat layer being applied.
It wouldn't hurt to contact a local custom paint shop and see what they have to say or sell (products, not services, per se). There might be workarounds as far as newer paint and pigmenting stuff, that won't compromise the integrity of the helmet.