dark brown or mahogany will do, komrad. if you're using spray paint, squirt the paint on to a piece of an old newspaper. do it close to the paper so there will be a pool of paint on the paper. dip a cotton bud tip into the pooled paint and brush the tip on your intended rusting surface.
have a rag handy so that you can wipe off any excess. oops, it's actually best to dab off any excess cos wiping can sometimes affect the surrounding area.
experimentation and some patience will give you the proper effect.
after you have achieved what you like, a bit of finishing coat on that area will help to keep the effect more or less permanent.
have fun!