Hero,
Don't know how much painting experience you have, but if you're asking questions like "what's primer?", it leads me to believe that your experience is pretty limited.
I would highly recommend searching for "tutorial" threads here on the forum to see how a lot of us have painted our armor and helmets.
The 2 basic methods are layered, and topical.
Layered means that you actually paint the helmet/armor in layers. Eg: put down a base coat of silver, then mask off the parts you want to remain silver, then paint gray (for the gray areas around the silver scratches), mask off again, and lay down a coat of green. When you remove the masks, you have a genuine "layered" effect.
Topical means working basically the opposite way. Your base coat would be green, and then you would add the other colors on top of that.
"Primer" is simply a generic word for an undercoating. Generally it's a neutral color (beige or gray, sometimes other colors), with a matte (flat) finish, and it's used to give the surface of an object some "tooth." In other words, it gives your overlying color something to stick to, as many molded helmets/armor are very slick when they first come out of their molds.
Another point to consider is washing the parts. A molded part will probably have some amount of mold-release agent on it. Just some type of substance that keeps the resin from sticking to the mold. Most people will wash their pieces before starting a paint job. I generally just use some liquid soap and a soft brush to scrub the parts down before I paint them. Seems to do the trick.
Most folks on TDH are really helpful, and like I said above, you can learn a lot just from looking over some other folks' tutorials.
Hope that helps!