From what I can find, the interior surfaces seem rather varied, but generally a rusty color, but not uniform.  I tried airbrushing light coats of Floquil rust and rail brown, but the results seem unconvincing. It seems like some sort of mottled or patchy effect is required. Thus far I have tried weathering the interiors of black hoppers, though many of my fleet, such as NYC, WM and PRR are a boxcar red color requiring something other than a rusty appearance to make the effect visible.