Here is the Sherwin Williams paint catalog illistration for UP boxcar. As noted it is the best record of how these cars might have been painted as a UP car.
Note... this is a scan of a color xerox of a catalog. The original (with its original color chips) is not currently available. This is a good indicator of the mid 1880's paint.
I think it is significant that Billmyer & Small is believed to have painted the trucks body color, and this indicates that UP painted trucks body color, so together these are best indicator of general practice - which appears to be trucks painted in body color.
Randy
Dave, I would be interested in your notes about early Kansas Central and Utah Northern cars...
Randy Hees
Director, Nevada State Railroad Museum, Boulder City, retired
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