After the "white armor" look of the silver leaf, this is a technique capable of more subtlety. I took a light gray armor and black grilled helm and spray painted them black. Why spray paint the black helmet black? To give the surface that is more receptive to the metal treatment - it is much more difficult to control on bare plastic, nor will it stick as well.
Plastic usually has some kind of mold release that needs to be cleaned off for paint to stick well. It's best to put the pieces to be painted into some warm water with some mild detergent. Let it soak for a while (I usually do it overnight, but I doubt it needs to be done that long). Rinse the pieces off, make sure they are dry, and the paint will adhere better. The first picture on the right shows a basic prepped figure, and the second picture shows the same figure after a coat of Rub'n Buff . Notice the difference?
Rub'n Buff comes in a tube and like the silver leaf (see picture on right), can be found at most arts and crafts stores (costs about ). It's comes in a variety of metal colors (silver, gold, copper, etc). It's basically metallic powder in a soft wax base that you apply sparingly with a cloth. I actually just use my fingertip (wash hands thoroughly and don't eat anything with your fingers afterwards). Let me reemphasize sparingly - this is kinda like dry brushing. You should not be putting this on like paint - your applicator (cloth, finger, whatever) should be barely stained. It's best to start with too little rather than too much. It's probably best to have some kind of sealant coat applied, but I haven't done that yet.
Finally, to protect your paint job from playwearing, a coat of sealer is recommended! Any types of clear sealer will work. I believe Citadel has matte, semi-gloss, and gloss spray sealers. I'd use semi-gloss, but it's a matter of taste.
Here are some more samples of Rub'n Buff armors. Notice the variety of metal colors that have been used (click on the images for a bigger picture):