You can use spraypaint if you know what you are doing.
First close all vents and remove the earflaps as well. Purchase some masking tape at the hardware store. Don't use duct or electrical tape instead, or your helmet will look like shit. Use the masking tape to over up everything that you dont want paint on, say the edge of the brim (on a 9.9 or BL). An exacto knife can be helpful in cutting the masking tape to cover things exactly, but remember that more tape is better than a little,because you can go back and fix it. Then tape the straps to the top of the inside of the helmet, so that they are out of the way and you dont get paint all over them which looks sloppy. Take some newspaper and go outside(so you don't stink up the house) in a dry place. It doesn't have to be warm, but if the paint job gets wet it will ruin it. Place the paper on a flat surface, and then place the helmet in the center the same way you would wear it, shell side out (there are some stupid people in this world). Then paint the helmet holding the can of spraypaint about a foot and a half away from the helmet. Go lightly and don't dump the paint on, otherwise you will get drips and the paint will take longer to dry. Sometimes it is best to do a few coats if you want a durable finish. Personally, I like Krylon Fusion paint for helmets, as it is formulated to stick to plastic and is quite durable. It comes in a variety of fun colors as well. After you let the paint dry for at least 30 minutes, take off the masking tape, and let it dry outside for at least another 30 min, as it still smells. After that, the paint should be dry and you have a new colored helmet for all your friends to gawk at .