On reading this enquiry I went straight to my copy of Boyd's "N.G. Railways of Mid-Wales" but there are no given details of interior painting, just of the external livery. My guess is that goods vans would have been left with a plain wood finish inside that would then gradually darken, with scrapes and scars from loading. The goods brake vans on the other hand could well have had a coat of paint inside - though not necessarily. I will try and locate my colour photos of the line (in preservation days) and see if there are any clues.
Meanwhile I seem to remember BR goods vans were plain inside, which may be the continuation of previous practice - perhaps others will have views, also.