The "Fleet" cars had been repainted by the time diesels pulled them.
Joe,
Several FOM cars made it into the mid to late 40s. It might have been possible, especially in 1946/47, that you would see an E7, PA, or Centipede pulling a consist of mostly FOM cars.
In a way, you are correct, but if they pulled FOM painted cars, it was for a very short time. Diesel power got off to a shakey start on the PRR when two E7s were made for the PRR in 1945 for use on "The South Wind" (a FOM route). They were side-tracked in Harrisburg, in part due to the fact that fueling locations were not yet in place along the line. They finally saw service in 1947, but the passenger cars were repainted, and given a blunt-tail observation car, in 1948. But like we often say, "It's YOUR railroad...run what you want".