Actually, for this particular car the "correct" answer is pretty simple:
"Anything goes, knock yourself out"
Based on the photos that I have of the 13000-series cars, some were delivered in all-mineral brown, while others were delivered in the AMC-standard "all black with dark brown sides". Oh: and those cars had black doors too, which don't seem to have lasted very long in service.
Lettering changed by 1940, with the road dropping the large corporate initials on the left side of the door, and with the semi-regular use of the "swing tail R" road name to the right. The cars began getting metal running boards in the late 1940s.
By the early 1950s, the cars were a jumbled mess. Some were all brown, some had black roofs and ends, and some even had bare galvanized roofs with brown roof ribs. The NKP was generally pretty standard about how they painted their cars, but these things got a little bit of everything. It's best to pick a specific car from a specific year, and model that individual car!
Let me know if you need photos of these cars; I've got 17 or so.
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