I've used Tamiya flat black, with a bit of flat tan, thinned with water until I liked how it flowed (I add a touch of dishsoap to break the water's tension). When I've done weathering, it's easy to over do it, so go slow. Think of how a real car gets dirty, usually the top gets rain, bird stuff, tree sap, etc, and the bottom doesn't get the rain etc. I've used more flat tan, or earth, or rust paint to add more tan to areas that are dirty, and more black for shadowy areas. Thinned flat grey can add a dusty look, too. Around the rust areas you have already, adding a very thin flat black wash with a touch of tan will add some darkness to the low spots, adding some depth. Take some scrap pieces, or a scrap body to practice on. Add the paints you did on your Mustang model, then practice with the washes. Once you get to the point you think it's almost enough, then stop and let it dry. You can always add a bit more later. I hope this helps!