Having grown in an era when you were instructed by kit manufacturers that all German aircraft were light Green and Dark Green on top and light blue underneath and all British aircraft were Dark brown and Dark Green (or OD) on top and light Blue or Light Green underneath, I have developed a great distrust for kit manufacturers colors and schemes given in the instructions. The only answer is to research the particular subject you are modeling.
The only problem is that compared to ships, planes and tanks, WW II military Railroading is somewhat lacking in available data. The only book I can remember near the subject is one published by Schiffer on German Armored Trains. You might try posting a request for information on WW II German train equipment over on the Model Railroader forum.
Having said that, I would suspect that Deutches Reiches Bann engines utilized by the military were painted much like German Military equipment - initially Panzer Grey, then Panzer Yellow w/Panzer Grey areas showing thru (areas not painted) and finally overall Panzer Yellow with Green or Brown splotches - either individually or together randomly over the Panzer Yellow.