My suggestion would be to pick out a particular color you like in the granite counter-top and use it on your walls. A beige color would work nicely to compliment it. Alternatively you could paint the walls different shades to try and give some symmetry to the room, or even decorate the two smaller walls with some brighter blue-grays and green-grays to try and make the room feel wider.
If you want to keep the colors of the bathroom pretty neutral, try finding a beige that is lighter than the counter top, but in the same color family. It would be helpful to go to the paint store and find lots of samples of beige and bring them home and look at them in your bathroom directly next to the counter top on the surface you intend to paint. Colors often look darker in the bathroom because there is not usually much natural light in bathrooms, so you want to choose the color under the appropriate lighting conditions. If you want to leave the beige color family, you could consider painting a the bathroom light-ish blue-green for a cool, relaxing feel or a darker brownish red for a warm feel (although going too dark might make the bathroom feel smaller).