In general there are two types of interiors: lined and unlined.
Most classic box cars after WWII were lined with plywood. Of course the plywood would exhibit streaks of fork lift tires or pallet scrapes et.al. I apply Tamiya XF-57 (Buff) or Tamiya XF-78 (Wooden Deck Tan) with a brush and the brush streaks kinda look like those scuffs. Some doors were also lined with plywood (such as many grain hauling 40' cars) but I saw many that were just plain unpainted steel.
The lined cars look either white or a very light gray. This lining is used in cars hauling food items and loads sensitive to damage or contamination, such as rolls of newsprint.
Plug door interiors were also coated in the white paint.
Modern mechanical reefers are a whole other story, but white paint would suffice.
