Paul,
In the late 1880s or 1890s car number 025 was still used more as a Business Car than a work train car.
So, I'd go with a UP passenger car green, whatever that was.
P-B-L, in their "Star" paint line, has a color called "SP/UP Dark Olive" (1-STR-29), evidently derived from Harriman standard paint specs. You might find that you like these paints, they are lacquers, have their own thinner and with the "Retarder" they flow out very nicely with an airbrush. You can find them on the PBL Online catalog at their website:
http://www.p-b-l.com/And don't forget the gloss finish, so nearby figures can be reflected by the car siding.
Jim Courtney
Poulsbo, WA