The cars on my layout all have metal wheels now, but it's been a long transition. In some cases, I've had to replace the trucks as well. When I've done this, or when I build a kit, I give the trucks a spray of rust-brown primer, a bit of powder weathering and then some Dull Coat to seal it. Those shiny black plastic truck frames are just completely wrong. I do keep the trucks on my passenger equipment cleaner looking, though.
In a nearby town, a homeowner in the historic district was working on his house, and got down to the original mustard yellow color under many levels of paint applied since colonial days. He decided to restore the house to its original color, but ran up against the Historic Commission, which blocked his choice of color on the grounds that "historic" buildings could only be white with black trim.
It takes an iron man to play with a toy iron horse.