From the very small number of B&W photos that reveal the inside of the wagon, the hue looks very similar to the outside. It also looks reasonably light grey from the odd color photo that had the same angle, although I'd imagine that could easily also be from the load (eg fertilizer).
Unfortunately the wagons tend to be tarped in most 'above lip level' shots . Steamranger have a shot of some Ys in action renal lasting the line, and the interior looks very rusty, but that may be a function of both the loads carried and a lack of repainting by AN over time.
Normal US practice was to not paint the interiors, which lead to them rusting very quickly.