I have opened my mouth here and got facts wrong so I will add these few observations as a layman and will happily stand corrected on any points of detail.
I have a pic in the book "The Darjeeling Himalayan Railway" By Richard Wallace which has a colour photo taken in 1989 and has the following caption:- "For a while in 1989 the North east Frontier Zone addopted a green and cream livery for the DHR's coaches" but of corse there were pulled by a blue loco. (I will happily PM the pic across but Lisa has warned us not to post copyright images on the site, and I do not wish to cause anyone any problems.)
I would think the green loco with the original DHR green livery (before indian railways took over) would mainly have pulled umber coloured rolling stock.
I have included a couple of pics I took in the Howrah railway museum (Calcutta) earlier this month.
Not the best shots. light was difficult sorry.