I suggest an overall "box car red."
I am no expert and only know of probably a fraction of standard gauge and narrow gauge ore cars and yet I can't think of any ore cars (except maybe small man-shoved size) with metal parts painted a different color than the wood.
The erudite and highly respected NEB&W Railroad Heritage Website records that when cars were brush painted and paints were primitive, that it was not uncommon to paint the iron black and the wood something else, mainly on reefers. Their article NEB&W Guide to Rolling Stock Paint Schemes - Box Car Red
) explains in great detail why "Box Car Red" was the most common train car color, and why B.C.R. usually isn't red as we know the primary color today. BCR can run from a bright red (starting around mid-20th century) to brown.
If you are not adverse to solvent-based paint, Tru-Color has many colors to choose from.