After removing the underframe and interior, then masking the roof, I spray the old Hornby Railroad coach body with Humbrol clear orange colour #1322. The number of coats will determine how dark an effect you achieve. So I suggest trial and error, and get variations from coach to coach. You can also use the Humbrol clear green #1325 to first brush touch over some panels, and this gives a brown finish when oversprayed with the orange. Even a try a tiny tiny touch of the clear red #1321 on some panels. This overall orange method means you retain all the lettering and lining as is. The colour photos of LNER well-used coaches indicate the overall effect was a brownish colour. After WWII, the LNER decided on a brown paint finish for steel coaches, to attempt a match with the older teak coaches. Teak was imported from Burma by most of the Victorian era railways, and without the high level of varnish, would have deteriorated to a silver-grey colour. The combination of dirt, coal dust, road grime and varnish ensured the overall brown look. I can only assume that some progenitors of the LNER (eg. the GNR) used the natural teak colour without staining. However the finishing of such timber took up to 25 separate coatings, including filling blemishes, sanding, and repeated varnishing. I do not know the chemical composition of the varnish. It was said you could see your face in the resultant finish, as though on a gentleman's carriage. (Our Iron Roads, F S Williams, 1883)