While it is against Federal Regulations to paint couplers and drawbars, it has not, historically, been vigorously enforced in all cases.
"Silver" pilots on Santa Fe, Western Pacific, and Burlington passenger diesels usually had matching couplers. Couplers on stainless steel passenger cars sometimes had silver couplers, if the railroad also painted the trucks and underbody components silver.
Overall, though, they are not painted. The steel castings rust at first, and then develop a certain oxidized appearance after acquiring exposure to the elements, acid rain, and the generally dirty railroad environment. They are usually a dirty brown color, not bright orange with rust.
In addition to the prohibition against painting, prototype couplers are not to be lubricated.