I’m thinking more of a car whose paint and primer are gone and you only see the raw metal.
There are a couple of ways. If you have an airbrush, you can make sure you put color coats on very thin in some spots, so primer shows through. If no airbrush, you can drybrush both primer and steel colors to get a translucent coating. In general, what you are talking about is weathering, and weathering is discussed frequently in the Techniques forum, part of the Tools, Techniques and Reference Materials section of these forums. Also, specific discussions of model car weathering are frequent in the Autos forum.
I dry-brushed with silver paint to represent areas where the paint was worn through on this North Vietnamese MiG-17F.
you can use two different colors of paint and a gentle sandpapering.
Here is an extreme case, but it illustrates the method.
I have a plastic Pierced Steel Planking aircraft base. It needs to be steel, with mud showing through the hundreds of holes, and along each strip’s joint line.
so, paint the base with my steel, and patches of rust, etc,until it looks like weathered steel planking before the mud. spray a light coat of the mud color all over the base.
then carefully and lightly wet sand off the mud on the high spots or anywhere else I don’t want it. everywhere I sand on the mud, the steel will show through.
after it looks like you want it, seal it with a clear coat (flat or satin, whatever you want)
it is a variation of the method used to paint the raised letters on real life engine valve covers, but, it works better and faster on model parts with hobby paints.
Rex