I have a Kadee #45060 N scale (pre-MTL split) 50 foot flat car lettered for DRGW #23015 and painted “mineral brown.” According to a 1970s-80s DRGW roster in Freight Cars Journal #23 (July 1989) p.11, DRGW had series 23000-23099 53’6” flatcars, built 1956, painted black with “Flying Rio Grande” lettering (and NO 50’ flats). My Santa Fe layout’s period is set in 1957. Would this DRGW flatcar have been painted mineral brown when new in 1956, or was this just a mistake by the model manufacturer?
It could very well be a fictional paint scheme. As you've already noted, the car body style is already wrong for the number series.
Considering my references are in a crate somewhere between here and North Dakota, I'll take a WAG on this one. I would assume the KD/MT car is not correct. The quasi-correct car is available from Athearn, IIRC. It doesn't have the number series you listed, but is a good stand-in until proven otherwise.
I mostly want to use the car I already have, nice general quality even if not quite correct for specific prototype. And I do not know DRGW prototype so backwards and forwards that it jumps out at me. I don't have decals to repaint. I am making a "good enough for me" compromise for the time being-- weathering the car with black so the lettering, and some of the color of the mineral brown paint, still show. If you don't have a lot of knowledge of specific prototype, it looks like a "typical" nondescript flatcar. The weathering jobs looks like it MIGHT be a brown car with a lot of soot, black weathering etc. on it, or it MIGHT be a black car with some rusty weathering. As I said, good enough for me for present situation. My standards may well be lower than yours.
I'll admit, I'm a bit of a rivet counter. Until proven otherwise, an incorrect car will bug the daylights out of me.