I would keep the color light because it's a lower level. I'd go something in-between light and dark, leaning towards light. You can use darker small area rugs to anchor areas if it's an open concept, or to create ambiance.

I would pick the floors first before deciding on your paint color. They are much more expensive and permanent. Pick light, medium, or dark depending on the style and mood you're going for. Next would be area rugs and fabrics. From there you'll be able to pick your paint colors (or perfect shade of gray if you still want it).