I think there are a lot of factors involved. First it has been stated a number of time by members of the Pennsylvania RR Technical and Historical society that Pennsy colors did change over time. It has been stated that they became darker and probably maintained their color better over time. The reason was the change in paint technology. Better carriers were used but the color change was also the result of the movement from natural pigments to artificial pigments for the color. So PRR tuscan red (I have never heard of tuscan brown mentioned before). was a lighter color around WWI than Postwar WWII. And then weathering and sun and use all affect color and fading and staining. Older paints tended to fade quicker and more significantly. So after a short while in service, the unit does not match an out of the shops unit. This is true of PRR Freight car color also. All freight cars and cabooses (cabins) were painted painted PRR Freight Car Color starting about 1920. None were ever painted a version of tuscan. Prior to that sometime - perhpas pre-WWI, cabooses were painted bright red - a version of Caboose red.
A story about this involving PRR Dark Green Loco Enanamel (DGLE). A friend of mine grew up in the Columbus Ohio area and saw a lot of first generation diesels as a young man and teenager. He said he always thought they were paint dark Blue! The sun fading and weathering and where he saw them, tended to give them a blue tinge rather than dark green.
Finally a third thought of mine. I worked in the steel industry a long time and then auto mfg. I was in a lot of industrial sites as an Industrial engineer. Industrial equipement was expensive and it was painted to maintain it mostly. some safety paint was done too. But the paint was put on to extend the life of the equipment, not make pretty. The company ordered paint from a supplier - often in 55 gallon drums (industruial stuff is big), and had it delivered. The guy ordering in purchasing specified a color based on a paint chip and followed the standard practices. The guys in the shop opened the drum, and put it on the machine, device whatever. They did not recheck the color or validate or even care what color it was. It was paint and it went on the machines - they applied it properly. I think the PRR as an industrial complex did the same thing. They used whatever paint showed up. It the mfg was a little off for some reason, the shop guys never noticed. Probably there was a little more notice for Passenger stuff, at least up until about 1955 or so, but it is still industrial stuff.
