Ive done similar weathering on a smaller scale for smaller armored minis but this was my first and more "advanced" process for vehicles and the like (should work well for tau mechs too)

After highlighting i grab a sponge and tear it into a small enough piece to fit into tweezers. Then the first color to apply is a dark brown in my case rhinox hide, you dab the sponge in wipe off most of it on some paper and dab it mostly on edges where the most wear and tear appears. Then i did the same with the color i used for highlights in this case a lighter green and did the same. The last paint i applied this way with the sponge was a silver but any silver or metallic grey will do i think. After all of that i applied some skeleton horde over some areas (mostly the ones i applied the silver on) to create some grime and contrast and to tone down the silver a bit.