I really haven't a clue
I would color your base fondant brown and use the airbrush for accent shading. It's super easy and fun. Use some scrap fondant to test first. I would use a darker shade but again, just roll out some scraps and test to see what you like. Good luck! I't'll be great!
I've recently obtained an airbrush and also have yet to use it. I am intimidated by the prospect of the mess in my cake room. I am wanting to make an airbrush box, similar to the one Earlene's husband made for her. Unfortunately I misplaced the instructions until just recently and now the cold, wet weather is making working outside impossible. I don't have a workshop so I guess I'll wait for spring.
If you are interested in making one for yourself, here is the link. Her instructions for the box are located near the bottom of the page. I think it looks really cool and I am eager to try to do it myself.