I've been doing the following for years:
About a decade ago, I had a liter of wall paint mixed to approximate the then current GW Graveyard Earth (which basically is a slightly desaturated raw umber). I use this paint as the base on the ground surfaces of all* my terrain projects, regardless of setting and background.
It is neutral enough to go with anything really; desert, grassland, jungle, urban; you name it.
The same paint I also use on all of my miniature bases. But I do make a slight difference between scifi and fantasy/historical bases; the scifi ones get dry/dead flocking/static grass, while the other settings get a more verdant treatment. The same is also done on all of my terrain pieces.
This means I can use the same tabletop with all settings, and can even change out terrain pieces without them looking out of place.
My method is one of the long view, but I'm pretty happy with it, as nothing is too specificly based to stand out on/next to any of my terrain.
* Not all; my recent arctic tabletop and accompanying buildings are completely snow covered, so these form the exception to the rule
