I have a D& RGW reefer (#43), the Rio Grande yellow colored-car. I had the roof ice hatches open and there was some light coming from behind the car. I could see the walls as translucent plastic inside the car through the roof hatch.
This bothered me so I carefully pried off the roof, there are four tabs at each corner on the long side of the car. Then I took black paint and painted the inside walls black. Any black will do, grimy black, engine black, etc.
When I painted the inside black I noticed that the grooved siding on the outside had a now very pronounced color change. The thinner plastic in the grooves on the siding allowed some of the interior wall black to "show through", almost as though I had washed the exterior of the car siding with a black wash. The tops of the 4 plastic slots on the car wall also need a touch of black paint to hide the R & RGW yellow.
The exterior walls now have more character and when I have the ice hatches open, I don't see a translucent plastic wall.