You can try a light brown weathering pigment. You dont have to buy from any special one. I do highly recommend secret weapon pigments, they are great. But if you are a budget go to your local hobby store (michaels, hobby lobby, etc) and pick up a pack of pastels (they look like chalk) grind up a brown, tan, yellow.. till it matches your basing scheme. You can simply use a hobby knife to scrape the pastels into a cup.
You can apply them dry (sort of drybrush technique) or mix the pigments with water and apply them wet. I would just play around on a test mini before going crazy. After you have the pigments applied and they are dry, make sure you give them a good seal with whatever sealant you use.
Hope that helps and or gives you some ideas.