Rendering color

Karl, I think you are correct, best locate locally a brick-built building that has been recently cement rendered and there is your colour. You are quite right too, when you say that the majority of buildings were left externally un-painted. Though there were camouflage schemes available in the early part of the war that show the preferred method of camouflaging a hut for example. We have one such drawing. We also have a complete file with hand-written lecture notes taken at a camouflage school with photos and official papers, paint test cards, plus docs showing how to paint airfields and factories. All interesting stuff!

Slightly relevant too is a drawing that we have showing how to protect wooden huts againt incendiary bombs using sheets of metal lathing (as used in plastering).

Just standard sand and cement mix which drys to a light greeny-grey colour but over time gets darker as it weathers (assuming its not painted).