I believe the upholstery was a dull bluish colour. It certainly was in CIE days but that could have been them putting standard Dublin bus style seats in it. I'm nearly sure I picked up so we here that CIE modified the seating at one stage.
The passenger compartment was done in a light colour, probably cream, maybe white. I doubt if it was the light green that the outer upper panels were. It's possible that below window level the interior was a darker colour but I doubt it.
The Finn Valley restoration of CDR Railcar 18 is not at all accurate in every detail, so I would not take that as an example. The type of flooring used in things like Donegal railcars was generally brown, and given that "B" was a Walker product, I would think it likely that this was the same.
Driving controls would not have been unlike any of the extant CDR railcars. It was often the custom to paint driving cab interiors in a dark colour on both early railcars and also buses. UTA buses had the same exterior dark green, or the lighter green used as a waistband, inside cabs. Belfast buses sometimes had brown. If you have any info suggestion the interior of the SLNCR's railcar(s) was / were brown,mid go with that, although the ones converted from old buses might have been the exterior dark olive green inside drivers cabs.