I assume this is a transition era car, like most BB kits.
Most probably, the railing would be sprayed the same basic color as the tank, at the same time. Then it would have been exposed to people using it wearing rough-fabric work gloves, which would have worn through the paint rather quickly at places where they usually gripped it. The borders of those places would rust, but the main grabbing points would be bright metal.
Then some safety wonk would decree that the railing should be painted high-visibility yellow. Noting the wear pattern, the paint shop foreman would specify heavy duty epoxy paint - which would not wear as fast.
Pick a point anywhere along that line, depending on YOUR modeled year...
Chuck (Modeling Central Japan in September, 1964)
Black tank cars almost always had black handrails, but cars with multi-coloured paint schemes might use a colour specific to that paint scheme, either matching or contrasting that on the tank.
The Athearn handrails are music wire, which is steel. If the rust is an issue, drag it through a piece of folded sandpaper to remove the rust before painting. For cars with plastic handrails, I usually bend new ones from music wire, as it stands up to handling much better than plastic or brass.
Wayne