A scheme I use for some of my Legions Imperialis armies.

1. Apply Vallejo ground texture. I like 26.215 Grey Sand
2. Base coat in Burnt Red
3. Optionally, dry brush with Dark Rust . It's mostly useful for larger rocks rather than small textures though. I wouldn't buy a specific paint for this; any black or dark grey you own would work equally well.
4. Mix Vallejo pigments 73.108 brown oxide and 73.117 Rust in a roughly 1:3 ratio, then stir in some Airbrush Thinner and apply as a heavy wash.
5. Once dry, paint the rim