I don't think rods were painted on most RRs. I think they were bare steel. Some surfaces machined and some rough castings. The rough surfaces tended to gather more grease adn dirt and look black, like between the raised edges. the smooth surfaces tended to be steel with a coating of oil to avoid rust. The steel color and the oil may look like a different color in differnt light
In my opinion, the biggest mistake that a lot of folks make when it comes to weathering steam locomotive running gear (rods) is, they are not really knowledgeable about what actually adheres to the rods/running gear. The forward portions of the running gear get the steam oil and machine oil thrown off the piston rod and crosshead guides, which tends to be pretty sticky stuff, even when hot. From the middle portion to the rear portions, the running gear picks up the white sand from the sander discharge nozzles, thus the wheels and rods pick up a very light color, getting heavier towards the rear.
