Hello,
It depends on the era and the railroad being modeled. FMC and PS cars from the mid-1970s through the 1980s were painted a buff/tan color inside. Prior to that most cars were primer or bare metal, and were lined with wood up the walls about five feet. Back in the wood car era, the interiors were bare wood.
As a side note, cars received in interchange, or cars that had passed through a yard would have all of the doors closed and locked by the car knockers. Cars could be refused at interchange or left at a customer's spur until the door(s) had been closed by the originating carrier or the industry that had emptied the car. Loaded cars would have the doors closed and sealed with the originating carriers metal seals.
Hope that this helps.