Muck
What colors should we paint our railroad car trucks and wheels?
Many variables to that question - environment, maintenance, era, age and usage. These examples are old Denver & Rio Grande wheels set into restored trucks. Look closely and you can see a variety of color and texture on the same wheel sets. Like the track, the metal wheels can even host the growth of moss/mold/mildew, as visible on the right wheel in the second photo below.
Two trucks of the same car (this photo and second one below...
...left wheel is much greasier.)
A variety of rusts and grime on these couplers
Rust patterns and colors on this bolster.