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I have a feeling - (may be incorect) - that the green cars came in the early 1960's with the purchase of the yellow/green diesels (GP30s). That was circa 1962.
The Reading painted its cars a dark brown, close to ATSF Mineral Brown. That faded to a redder color. I like to harrass my ATSF modeler friends that ATSF Brown should be called P&R brown since the P&R is older than the ATSF.
Reading reefers were painted white and there were a class of early wood boxcars (1900-1905 era) that were painted a dark green for "Provision Express" service (Clover House makes dry transfers for this car and the car looks like a Westerfield XMh car only 2 feet shorter).
Early (1880-1890) line service cars (Central States Dispatch, Red Line, White Line, Blue Line, etc) were often painted different colors (CSD cars were yellow and Blue Line cars were blue).