This would mean moving the period from the 1930s to somewhere in the region of 1913. Ultimately this means that the line could have been operated by the North British Railway. There seems to be surprisingly little information recorded about the NBR in a country that has managed to preserve plenty of history about the other lines of the era.
Paint is a real issue, there were several different liveries carried by the NBR, but periods that these were used over seem rather vague, and there were some locos that overlapped from one period to another anyway. Phoneix paints produce some NBR colours, however I find them difficult to achieve a good finish with compared to other brands of paint.
Instead, from looking at photos of a preserved NBR loco (Glen Douglas), I found Citadel Colour 'Gretchin Green' foundation paint.
The colour is a close enough match for me to be happy. Now to try and sort out how to do the lining!!