There are precious few possibilities for 28mm Tibetans sadly. On the plus side, if you trawl online you will find numerous photos of regular Tibetan soldiers of the rough period - and none of them ever look the same, so the world's your oyster! Sometimes wearing pith helmets, sometimes wearing oversized fur hats - possibly summer/winter issue although it may just have been officers who got pith helmets. They also even wore those Australian slouch hats pinned up on one side. In terms of uniform colour, that too varies - sometimes a light khaki, other times looking more of a dark green like the Gurkhas (tho difficult to tell from B&W photos).
PS would be interested to know if the Miniature Figurines Tibetans are the same size as Copplestone range. Anyone know?