Blue

depends on the tint of the travertine,, it comes in a million light shades. . .

But i think you are leaning the wrong direction. . Neutral doesn’t necessarily mean cream color or celery tones. . .

The travertine is the bell of the ball in your room and to REALLY make it POP. .

You need a deep dark wall color. . Dark walls in bathrooms are becoming very very popular. Go darker then you would ever imagine and your tiles will look as if they have a spotlight on them and the gradients of the natural stone will come out. .

If the tint of the stone is in the cool range, go with a stunning gunmetal blue dark paint or a lavish hunter green, and if the stone has a hot contrast in the beige/orange/redish tints of white. . . Nail a wonderful deep chocolate up or a rich burgandy. . . Be BOLD and your bathroom will sparkle like you would never imagine. . Good luck

to make my point

imagine this picture with the walls painted the color of a dark chocolate bar. . Would be stunning. . That beige is dreadful. .
-http://www.Servicemagic.com/resources. Dg. Project. Bathrooms. 4. 7. 81738.html

What colors do the tiles have in them? What color is the vanity? What color is the floor?
Go here and try some colors out:
https://www.Sherwin-williams.com/visualizer/
http://home-and-garden.

Webshots.com/photo/2419915660058499758edXGqf
Try 2803 rockwood terra cotta.

Wow , I agree with Jason. But I wanted to add- be sure to have good overhead lighting. And with the bold colors try to have lots of silver, big mirrors and glass in the room for sparkle. The more light you can bounce around the richer the finishes will look