One of the nice things about paint is that it's relatively inexpensive - if you don't like it or the future solatube doesn't flatter it, change it. Having said that, I'm having doubts about your color scheme. You're describing a white/near-white room and fixtures with one dark element, the vanity. Even the towels are vanilla. Have you found a shower curtain that picks up on that expresso? Dark framed mirrors or wall art? Washcloths? If you have those dark elements somewhere (hopefully not all on the same wall), then you're probably OK going with a soft white on the walls. Choose one that looks good now and change it down the line if it's not so good with the new solatube. As far as choosing the "right" white, see if your paint store has trial size cans and just paint a swath. Failing that, collect up several color strips of the color you might like, cut out the one you want to see from each strip and tape 9 or so color squares to the wall to see what it looks like. P.S. As long as you're trying out whites, why not make a patch of red or another bold choice and see how it looks on a wall or two? You never know.

I have gray wisp in our bedroom and throughout the house. Love that color! I have dark brown bedding with seafoam pillows and lamps. Here's a picture although the lighting isn't great. I also think a rustic red-ish color would look great with it and that's what I plan to do with our living room eventually.

Yes, two shades darker will look very nice but subtle. Like Scarlethouse says, going just one shade darker will not provide enough contrast. I'm the opposite of most, I still happen to like accents walls. I think they can add interest to an otherwise bland room.

Depending on your natural and installed lighting you might choose something from the whites collection of your brand of choice, or you might go into the blue/grays of a collection. Do you have any natural light? Are your light fixtures installed yet? You would want to have your fixtures installed and the light bulbs in before choosing the color. If you are doing whole walls, I'd go light. But if you have something like white painted wainscotting on the lower walls and drywall above you can go bolder/darker. A lot of it depends on lighting. Could you post pictures of what's going on in the rest of the room?