-Rust-
first step, prepare the area you want the rusted effect with
Skrag Brown, apply it in a relatively rough pattern where needed. i would recommend doing this BEFORE detailing the rest of the model, as clean up of the surrounding base colour will be needed.
next, apply a finer line within the Skrag brown areas, using
Fire Dragon Bright, you'll want this to act as a ''highlight'' to create the colour differences in the next stage
third, wash the area down with
Agrax Earthshade, use as much or as little as needed to get the effect you want, though i recommend starting light and darkening as needed
once dry, apply edging using
Runefang Steel, applying darker metallics such as boltgun metal and working up through chainmail and mithril also work pretty well, it's really down to the effect you want. it also works well with golds though it can often just blend in with the oranges used in the surrounding area.