Here are a couple great ones- I would go with a warm gray. Revere Pewter & Hot Spring Stones. Hot Spring Stones has the slightest violet undertone and looks great with black and white. R Pewter is a designer favorite. Good luck!

I posted a similar question and some people recommend not doing a light color in a small windowless bathroom. However, I did just that - I used a beautiful white that has a tinge of warm pink in it, and it looks fabulous with my wood floor, calacutta tile around shower, and white fixtures. It's Benjamin Moore Atrium White. It's a clear but luminous white...the creamy whites looked terrible in there. Be sure to test whatever you're interested in because your lighting, tile, floors, etc. will effect how it looks. I think I looked into or tried every white BM had, LOL. Good luck!

Don't feel like you need to limit yourself. Powder rooms are great places to be bold and try new ideas. They can really be anything, from super dark to a funky patterned wallpaper.

You can also look for a wallpaper that represents one of your interests. We could not have children, but have had a succession of dogs, so this one was our version of whimsy.