Passenger car condition varied greatly depending on era, location, and car type. Passenger trains and locos were kept pretty clean prior to WWII. Less cleaning occurred during WWII when car utilization was very high. After WWII, most, if not all, passenger trains passing through Sunnyside Yard in New York when through the car washers. Lightweight cars with their smooth sides were easier to clean. Heavyweights and older cars often were weathered by the elements and lost their gloss coat. P70 coaches looked shiny and clean when freshly painted, but older weathered cars were more common, particularly in the last decade of the PRR. P70s in commuter service probably got dirtier quicker - think Pittsburgh commuter service - but New York and Philadelpia cars probably ran throught the car washers more regularly.
If you look at prototype photos, you will see that most PRR passenger trains were a mix of lightweight and heavyweight cars of differing weathering. As cars were shopped they were repainted, so there could always be a mix of shiny, freshly painted cars, in consists with dull, weathers cars.
To answer your question directly, it is hard to go wrong with any combination of finishes and weathering. What was probably rare was an entire train of shiny new cars.
