A picture or two might help.
But going off of the description, I think ochre would be a safe choice. It's both a lighter color and helps buildings stand out sufficiently, and you can use darker brown in recesses and around the base to tie them to the board. plus as a fairly neutral color, you can add a number of spot colors if you wish without worrying that they clash.
I occasionally play on a black/dark grey board with black/dark grey buildings at my local store. It doesn't look terrible or anything, but it can blur into each other. Personally I'd avoid this.
If you want color clash, you could go for white. Since you mean to build ruins there's plenty of room for weathering, battle damage and generally darkening them down a bit so they don't stand out quite as much as they could, but you'll definitely get large areas that will stand out.