Here's some options.
Paint it entirely body color, which if you watch barret Jackson, is widely done.
Or, to represent the factory look, primer grey or black with body color overspray like this. http://www.apexautosports.com/2012/fresh-overspray/
However there is really nothing wrong with what you already do. A semi-gloss black chassis with a silver gas tank really looks sharp in my opinion. It makes everything that isn't black, really pop.
Another thing you can do no matter how you paint it, is to add some road grime.
Plenty of options. Just have fun with it.