Dip the end of your small paintbrush into a container of rust-colored paint. Dab the paintbrush onto the object to create small patches or dots. Leave space in between each blot.
Swipe a dry brush over the paint quickly and firmly just before it dries. Your goal is to push the paint around a bit. Doing so will create a graduation between the object's original color and the rust color.
Dip your paintbrush into another shade of brown, blotting the paint on with the same technique you used to apply the initial color. Apply the paint to the unpainted areas. Brush the paint with a dry brush just before it dries. Continue this process with other colors that are similar to those you are using until the entire surface is painted and has achieved the look you desire.