Having spent a good hunk of my time as an A&P working on and around warbirds I'll put forth some observations. About the only time the exaust systems look metalic is when they're brand new. And then they have a wide range of colors - dull silver to almost metalic black. After a few hours of operations, they start to rust with the colors ranging from a blackish brown to a dark orange with out any sort of metalic coloring. Suggestion, if you have a car that is more than 2 or 3 months old - go out and take a look at the exhaust system under it - take away any dirt and it will pretty well match what is on an airplane.
I've got Rust color in MM, Humbrol, Testors (small bottle) and even a couple of bottles of Pactra. They're all a slightly different color (tone?) and all get used on a regular basis on the exhaust systems or my models.