Just to expand a little on the information from SR man. The timbers used to veneer the interiors were typically tropical hardwoods, and very attractive when varnished. Depending on the timber used, anything from a light flesh tone to a rich mid brown could be seen. Narrow (roughly inch wide) chromed strips were used to cover panel joins. The interior window fittings, door handles and other metal components for tables and the overhead luggage racks were also chromed. Ceilings were a rich cream colour, floors various shades of grey and brown. Upholstery was in moquette, two colour combinations stand out in memory: a fairly dark red with a woven black and grey pattern (2nd Class) and a deep blue with a woven mid green pattern (1st) usually with an antimacassar on the headrest.
If this is OO then 'scaling' the timber grain would result in fairly uniform tone, and the chromed cover strips would pretty much disappear. It's neat to have a bit of variation in the interiors from coach to coach. I think it worth painting the seats, and adding the antimacassars in first - a pale grey looks better then white by being more subdued. I have a buffet done with the 'plastic interior' as I remember one such from my youth, pale cream and grey interior, but no claims for anything remotely accurate!