I used to paint my helmets using masking tape & aerosol paints. This is the same Bell Star II (apologies for the top photo, but it's the best I can make it

). A proper spray gun is preferable- I found aerosol paints to be pretty 'soft' after they've dried.
The paint on a helmet gets scratched & chipped over time & I used to repaint it every couple of years. This old Bell has been retired for over 20 years!
DaveB.