Tough to find the right gray when you have travertine on the floor.

What about something really different? Like pulling one of the rich dark colors from your travertine? You have a lot of good light, and nice high ceilings, so I think your space could handle it. It would be high drama and a give nice cozy feel. Definitely would make a statement. If you don't want to do that everywhere, painting just the ceiling in a dark color could be an interesting choice too.