Is there a colour in the adjoining room/hall that you'd like to pick up in your powder room?
You can keep it neutral (beige, tan, cream) or go more colourful. A stunning wallpaper could be nice.

Have you browsed the Houzz photo section for inspiration?
https://www.houzz.com/photos/powder-room-ideas-phbr0-bp~t_713

A more creamed color would help warm up the powder room.

Thanks!

Adjoining hallway is just white. The living room past the hallway has a dark blue (almost midnight) accent wall.

I don't mind a "strong" color but worry that a purple for eg might be too overwhelming, while a cream might just be too plain.

Any more thoughts? Paint has definitely been the most difficult decision process in this entire renovation so far!! :-)

A darker cream, like the Tan suggested by Dytecture, or a taupe colour could work very well. I'm assuming you'll have white porcelain in the powder room? That will look very nice. You can dress it up with a beautiful light fixture, mirror....
Or a really interesting wallpaper, like an architectural wallpaper that will add depth or a floral one. I found a couple of examples for you. The architectural one that I was thinking of doesn't have the monkeys (unless you like the monkeys), but that additional pic gives a good image of what it would look like.
Here's the link to where I found the wallpapers. They have terrific options, endless hours of browsing fun ;-)

http://www.fabricsandpapers.com/item/view/11028-Cartophilly-wallpaper/category/theme-wallpapers
http://www.fabricsandpapers.com/item/view/11102-Procuratie-Wallpaper-Black-Stone/category/contemporary-wallpapers
http://www.fabricsandpapers.com/item/view/8881-palmnava-wallpaper/category/contemporary-wallpapers

This is the paint colour:
http://www.sherwin-williams.com/homeowners/color/find-and-explore-colors/paint-colors-by-family/SW7549-studio-taupe/

Hover over the collage above to enlarge is.