Ah, it is outside ! So diorama is in front of real, purple/green land in background...Perfect effect...This type of setting is usually what I have in my mind's eye when thinking of how a MRR should look.I.E. Most MRRs have a painted on mock up of mountains/hills/clouds/sky that always look too clear, too ' in your face '..and actually take away from or upstage say, the principle characters, the trains them selves..The hazy, foggyish, maybe even rain clouded background really captures the essence/environment of RRing....Fabulous...And, it's Z scale ?!!!..Sure you didn't just flip an N on its side accidentally ?
FLG - The closest I've come is dryer lint which is (from my machine) almost the perfect grey color. But even this (at least to my eyes) just doesn't quite make it.
