I agree with most all comments thus far...
For the true "museum-shadowbox" look, most definitely go with the flat black!
On the other hand, I prefer an "almost" to that contrast and went with a darker olive drab "Cambridge Green" from the Dutch Boy chips, for both my fascia and the valance.
But in all honesty, the big thing with what ever color chosen, at least in my mind, is that it is a FLAT finish. A flat finish will not pull attention away from your pike with its WOW of sight!
I also strongly believe that this should be the case with a backdrop. Yes, I have seen some REALLY nicely painted backdrops with both landscape and detailed scenes.....But I still feel that a flat, less detailed backdrop does not distract from the diorama. As should a flat, neutral color on fascia and valance.
Puffin' & Chuggin', JB
Chief Engineer, Colvin Creek Railway