What paint color for the base?

No sense arguing with Mother Nature. Start with some shade of "dirt" mid-brown and work up from there depending on what you want the scenery to be. 

If you are going to apply full ground cover, the best color for the base is flat black. This is especially true for mountains and other non-horizontal surfaces. The reason is that you are going to want to simulate deep shadows behind your applied textures and colors. It is much more effective to have them there from the start than to try to add shadows later.

If you are only going to apply ground cover "here and there", then earth brown or grass green are better choices.

Thank you. Always nice to get some insight. The black will definitely work well on my Rocky cut thru. 👍🏼

I used a basic green for most of the layout. A basic brown for the western area. I will add pics and my ground cover pallet.

I used a very dark grey and lots of ground cover. One day i took an empty paper towel tube, closed one eye, and looked around outside. The predominate color appeared to be grey.

Fort Pitt Highrailers have a light brown latex paint that we use on the Modules.

Edit Add paint mix.   I took a small part of one of the modules to the local hardware store and had them match the paint color.   This IMO is a pretty good base under most any scenery detail.

Benjamin Moore & Co.
Interior/Latex/Flat N215   Regal Premium Flat.
Base N215-3B Quart
BK 6 1/2 shots
BR 2 3/4 shots
OY 13 shots

I also used a medium dark brown latex paint from Lowes as my base. About the only difference is I sprinkled green and yellow turf blend on the paint while it was still wet so it will adhere to the paint - seems easier than doing it afterwards. You can just set buildings and structures right over it. I also masked off any roads I wanted and came back later and painted them a dark silver/grey and used a white paint pen to draw road lines.

black