Run your fingers carefully over the surface of the helmet. Can you feel any bumps or ridges where the edges of the different colors are? If you can feel anything, it will show, even if you use primer over the whole helmet.
It would probably be better to at least give it a light sanding. If you use a fine grit sandpaper, like 400 or higher, it'll smooth out those transitions between colors. I don't see any reason why you'd have to sand down to the bare surface.
For misting, you'll basically take a spray can (or an airbrush) and hold it pretty far away from the helmet. Depending on the conditions, the distance will vary a little. You're basically letting a "fog" of paint settle onto the surface. If you don't have a lot of experience with spray painting or airbrushing, practice on something first. A piece of white plastic and some black spray paint will give you a good idea how the effect varies with distance and duration of spray.