Personally, I paint my Gurkhas in rifle green, since I think it makes them stand out, and to me it's the "iconic" Gurkha uniform, but as shown in the collection of illustrations, you can really take your choice amongst all khaki, all rifle green, or the less-common but still legit khaki tunics & green trousers. Same for khaki or green or black puttees OR black leather gaiters. Whatever you think will look best.