Okay, time for the hive mind of WT to come to my aid.
Seat colours. Now, having read all I can about the period (late 1880s) I believe first class seats were upholstered in red Morocco leather, second class in a dark blue rep (possibly with a floral pattern). New build first class for the narrow gauge apparently was upholstered in crimson fabric of some kind. For the third I built last time, I think the seats were painted to match a brown fabric, or was it green? Ah, memory! How you fail me!
So, bearing in mind I'm not adding button details, I don't plan on reproducing a floral pattern either! Does the team think a deep maroon for first class and a deep navy blue for second would be adequate?