According to TLC's book "Atlantic Coast Line Passenger Service" by Larry Goolsby, the C-6 scheme (aluminum-gray band above the windows and purple below with black roof and underbody) was first applied to passenger rolling stock in 1950. These were class A-15 coaches which were being rebuilt to lightweight coach standards.
Sometime in the early 50's other conventional heavyweight cars began to receive this paint scheme as well, but some of the cars never did receive the purple colors.
Beginning in 1957 the purple cars reverted to Pullman green as its standard, although at least one car retained it's purple color just prior to the eve of Amtrak.
Most of the ACL Pacific's (4-6-2's) were removed from the active roster by the end of 1951, although engine 1735 lasted until May 1953. Therefore, there was an overlap of a year or two when I guess it was possible to have purple passenger cars pulled by steam.
The above information applies to conventional equipment (heavyweights) and not to streamlined lightweight equipment. Hope this helps.
Dale Latham