There are as yet only two locomotives on Unnycoombe. The 57xx Pannier was shown in an earlier post, the other is a Dapol Prairie both, baring in mind the period modelled in early 60s, are pretty grimy. The Prairie was still in it's pristine livery until yesterday when I attacked it with acrylics and some coal dust. Both the locos, and in fact most things on Unnycoombe, are referenced with regard to colour from an Atlantic book: The Colour of Steam Vol 3 Great Western Branch and Main. (ISBN 0 906899 16 8) by P.B. Whitehouse. I've had this book since I was in my late teens so it's probably out of print by now. What it does show is that during the period 1961-5, unless on top-link passenger duties most stock was pretty sooty and uncared for. It was this feel that needed to be replicated for Unnycoombe and this Prairie was influenced by a plate of 4161 exiting a tunnel near Gloucester with a three coach train; a scene not dissimilar from the arse-end of Unnycoombe.
Again, I have to admit that wethering N Gauge stock is nowhere as simple as it would be in 4mm.