Vallejo Smoke is my go-to for making anything look worn, rusted, or leathery. It's not an ordinary paint, it's almost a technical effect, being gritty brown flakes suspended in an orange medium.
Here's an example of some models where I used a thin wash of Smoke on the metallics to give them a dirty, brownish tinge. The ratio here was about 4:1 Smoke/water, so a thicker mix will produce a more pronounced effect, and you can vary the dilution to adjust the intensity of the effect.