As I cannot get Precision Paints, etc. in the USA, GWR colors are always a bit of a problem. So, permit me to suggest (your opinions appreciated) that Vallejo Model Color 70.843 (cork brown) makes a pretty good GWR Medium Stone and two parts cork brown + one part Vallejo 70.940 (saddle brown) makes a pretty good GWR Dark Stone. Looks OK for my platform lamps and platform fences. If anyone tries this, share your opinion because this is a really easy way to "approximate" GWR stone.
George
North Carolina
GWR "stone"
Re: GWR "stone"
They are unavailable here in Norway too. Vallejo Model Color 70.843 (cork brown) was a very good match on my Bachmann Highley signal box. Thank you for sharing this tip! 
Carpe Diem!