Thanks. I actually did some more research into this. Pullman Green became standard in 1900, replacing a dark chocolate brown color. In 1909, PRR agreed to place massive orders, but demanded everything be done in Tuscan Red. By 1937, Pullman itself was manufacturing five different paint schemes: Green, PRR Tuscan Red, Wabash RR, Milwaukee Road, and Tennessee Central. By 1942, this number had increased to twenty one, and by the 50s they offered over 100 unique livery choices, tailored for different railroads.
Up until the 30s, most railroads used Pullman green. This appears to be mostly for brand association, as they wanted passengers to know that they would be getting the authentic Pullman experience and service while riding with them. However, the Pullman green was slightly different for each company, because after Pullman built and sold the cars, the individual railroads were responsible for repainting them. Each railroad in turn would make and utilize it's own formula of Pullman green, close to, but not precisely the same, as the factory-fresh originals.
Railroads that didn't: Wabash was using a dark blue from 1925 onward. Pennsy as mentioned did Tuscan Red from 1909. Milwaulkee Road was using maroon and orange from 1927 or possibly even earlier.
I initially asked this question because I'm freelancing a class I railroad in New England circa 1924, and would have preferred to use a grey color for my passenger cars. I wanted to see when other railroads had begun to differentiate themselves. I'm going to go ahead and do so, justifying it my railroad being successful enough and with a large enough fleet to make such a distinction worthwhile, wanting to set themselves apart from the B&O, NYC, and NYNHH.
Lastly, as an academic point, railroads began to switch to their own livery at about the same time as colored printing and photography became more common. I wonder if they did so specifically so that, in photos, they would appear unique and recognizable.