My recommendation would be to find a powder coater that will work with you on piggybacking it with another client who is running that color in large volume. It takes quite a but of time and cost to purge the spray gun for a color change. To do so for a few train parts is going to be quite costly. If they are spraying a bunch of stuff, throwing in a couple small items will be a pretty cheap add on. It could mean waiting a while though, especially if the color you want is not in common use.
My previous job utilized a lot of powder coated steel parts. We did a lot of black but had some parts that had to be red so they saved up the red until a large batch could be run. Doing any other color for a special project got expensive.