Contact the manufacturer of the helmet, most have a web site, and a way to contact them. But I'll bet they say it will void any warranty that is in effect(if any) if you paint the helmet, with any kind of paint.
I'd get a better helmet, with bigger vents.
And I'd cut the hair. I did.
I was riding on a 90+ degree day last July, I was hot, and close to home so I squirted some water from my bottle on to my head. The water didn't make it through the matted, sweaty hair. I went home, showered, got in my air conditioned car to the local cheap hair place. The nice girl asked what I wanted, I looked at her and said "number 4". She told me it would be pretty short. My response was, when you're done all I want to see is a pile of hair(on the floor). Buzz, buzz, buzz, clip, clip, clip, and done. It made a big difference in cooling to have the air flowing over my now short haired head. Plus it grows back.