I assume it would be a polyester resin based gelcoat on the helmet.
The manufacturer would recommend against it, but should be fine. To make the paint stick well I would sand the area to be painted with 120-180 grit sandpaper, otherwise paint will peel. Over time the paint may flake, but by that point you will have retired the helmet.
The best option would be to paint with poly gelcoat. Try the tintable options at Jamestown Distributors Boat Building and Woodworking Supplies
or use krylon spray paint.