Michael is quite correct in regard to commonly available model railroad paints. Floquil rust does a nice job of representing fairly new rust, as might be found on say the roof, or sides, of rollingstock. Floquil Rail Brown is a good choice for a weathered, dirty rust, like that seen on freight car wheels, trucks and couplers. Of course, the most accurate representations of the various stages of rust are created through using these two paints in combinations and sometimes even with some Reefer Orange to boot.
For what it may be worth (and everyone has their own opinion), I personally don't find the corresponding Poly Scale paint colors do quite as good a job as does Floquil.
NYW&B