I recall once at a worksession that the D&RGW refers were either natural or red inside. But then it was dark and I was in the ice cage retrieving the !@#$%^& wrench I dropped down there while installing the hatch!
I believe that boxcars were natural wood--there was probably no reason to paint, and I suspect that refers are the same. But there are other heads out there that know FAR FAR more than I.
Keith