That would be a challenge. I've seen some black and white photos and it looks like there are some differences in the colour tones of the adjacent boards, I assume because of underlying wood. Maybe using 3 or 4 slightly different 'varnished mahogany' tones applied to the individual siding boards. All of the photos also show these cars spotlessly clean, so a good gloss finish, or maybe 'just off high gloss' would look good. Some of these cars also had a lot of gold leaf pin striping and other decorations; I have no idea how you might go about simulating that. I've never seen a colour photo of one of these cars as I don't think the colour photography had been perfected when these cars were in general use. I would be surprised if a museum somewhere hasn't restored a car of this vintage. I bet the Railwire knows...
Geoff