Welcome to the site. I look forward to your models.
Unfortunately, I did not find any color photos of German armored trains that were not painted dunkelgelb c. 1943.
Before then, the trains were painted by Deutsche Reichsbahn
(DRB) specifications, not Wehrmacht. In fact, when DRB decided to camouflage, it was the Luftwaffe that collaborated with colors, not the Heer.
I believe it is fair to speculate that a dark gray was used. Locomotive classes 42 and 52 were originally finished in "Wehrmachtgrau", officially known as Eisengrau (Iron Gray) RAL 7011. There was also RAL 7021, Schwartzgrau, (Dark Gray), also known as "Panzer Gray".
There are references to "Eisenbahngrau" (Railroad Gray) but I haven't found any RAL references, so I think it was a railroad term for RAL 7011 or 7021. "Eisenbahngrau" could be confused with Eisengrau; this could be the same slang as "box car red" - probably a common red preservative paint (Red ochre—Fe2O3_) used on barns - but renamed by railroaders.
When they started camouflaging the trains, the directive "Munstertarnplan III" was issued. It specified ratios of two greens and a brown, with a very small amount of "sand" - not dunklegelb - over the gray, in about a 1/3 gray to 2/3 other colors pattern.
An author named Wolfgang Sawodny has written at least 3 books: Deutsche Panzerzüge: https://www.amazon.com/Wolfgang-Sawodny/e/B00J5Y0R0U
I hope this helps.