For a fairly quick, overall weathering technique, try this:
An acryllic wash with suitable earth, dust color. ( enamels work just as well)
Remove or cover helmet liner.
Drop a few small nuts, bolts, screws or similar into a coffe can (or similar container) -remove coffe first!
Add small amount of dark brown/rust colored paint (color as desired) Just enough for the screws etc to pick up some paint.
Put helmet(s) in container, secure lid and shake.
Allow to dry
Repeat the process with silver (or similar) paint.
I'll try and post a picture later.
Graham