Weathering depends on what you do.
If you want it to look like it has a thin layer caked on, get the yellow and browns you want, mix with water or appropriate thinner, and brush it on towards the top of where you want it. It'll run down the rest of the way, leaving the drip effect. This method is also useful for little splashes of mud on the armor. (Just get your brush wet and flick the bristles back with your thumb. It should send a good spray of big drops out.
If you want heavy amounts caked onto areas, you'll need probably a good clay. Just cake it on there, scrape bits off, and make it look like dirt and mud clods. When you're done, be sure to paint it a rough brown. Mute, dark brown works best. (To the point where it's almost a textured dark grey)
Maybe slap some clay on an area, then take a stick and pry the clay out of it, haphazardly, making it look like the armor was given a rudimentary cleaning.