What's the best color for the interior of a studio?
I would think that the backdrop is very important, so maybe the color of the walls isn't a big deal?

Whenever I can, I prefer to use available light from two windows facing East and South, rather than studio lights. I'll need sheers, diffusers, or shower curtains until the sun gets below the tree line.

North-facing windows will give you more consistent lighting throughout the day...

The wall color should be neutral. Either white or a neutral shade of gray.

Do you want light reflecting off of green walls onto your model?

Light or medium gray.

White is too reflective and makes it hard to control contrast on your subject. Black is just depressing, in my humble opinion. Any other color will reflect onto your subjects.

Agree with K Bennett.

Black (especially combined with high ceilings (eg 15' or so)) would be ideal, but far too depressing. With reasonable size a mid grey minimises reflections, and you can always add light to it to brighten it up, if you need to shoot against a wall.

Even seamless coves (aka Cyc's) sometimes get painted from white to grey for the same reason.

Hope this helps