Val,
Weathering of a locomotive, depends on a lot of variables. First, the railroad. The locos of the C&O got dirty, but they started the day clean. Because they were "cared for", they didn't get much visible rust. What they did get was dust, soot, and if they worked grades, sand dust.
Type of service. As mentioned, locos working grades picked up a lot of light colored dust from all the sand on the rails for traction. These engines would also have a lot of soot buildup by the end of the day. If the locos weren't cleaned regularly, the dust and soot would soak up the lubricating oil in the steam, and the dust and soot would take on a wet appearance. Locos working as switchers, would not get the "sanding", and the frame, and running gear would be darker. Switchers might see a more regular cleaning, and might even retain a shiny appearance.
Location. The kind, and colors of dust would be determined by the types of soil the right of way passed through. This could go from a light tan, through dark gray, to brick red.
Unless maintenance was restricted by cost, most railroads kept their locos relatively rust free. Towards the end of the steam era, as railroads shifted to diesels, even roads like the C&O, let their steam locos acquire a more dirty, and used look, as the need to maintain them fell off.
Check out Shamus' Badger Creek. You'll see that the enginehouse has a loco cleaning facility just outside the doors.
White tires/rims. The photos I have of the Delaware & Hudson steam locos, show that white tires were common to this line. The photos also show that a lot of care was given to maintaining their locos. They aren't heavily weathered.
Hope this helps,
Pete
Val
