The same paint that made the helmet in the first place is what is in the re-painting. Mind, you want to tape off the openings at the neck and face shield, just use masking tape and newspaper to make a barrier. Not hard.
You'll sand the helmet with 400 grit wet/dry sandpaper with a little soapy water, no need to go through the color. You just want to scuff the clear coat.
Then you can use a very good quality color (I like Duplicolor available form auto parts stores), follow the directions on the can for 2-3 coats.
Then, I have always clear-coated the helmet with either automotive grade stuff or with the spray can stuff. The equipment from Harbor Freight for the auotomitve grade stuff was less than $70, the clear coat itself doesn't cost much either, just ask the guys at the paint shop what you need and what to do. It's seriously easy. The prep work is far more involved than the actual painting, painstaking cleanliness like you'd do with preparing food for your family. I've painted two bikes, not just helmets, and had good results just by asking lots of questions and watching it being done.
I have painted all my helmets, one for each bike, since it's so hard to find a helmet to fit my tiny head that I buy whatever color fits me that I can find.
You won't make a mess of it if you can successfully paint your toenails....