Yes, Grey or a soft Blue - maybe even a glass subway tile

I like the idea of gray or a soft blue, white would also work. Look into a larger subway tile, a 4x12 or 4x8

Go with something to accent but contrast the rest of the kitchen.

Do you want color or do you want the look to be monochromatic? If you want color, anything goes. A larger beveled glass subway tile would look great. You could choose any color, depending on what you want for the final outcome. But your floors look on the warm beige side, while your cabinets/counters are cooler tones of gray/black. Personally I'd add a bright color to balance the difference in tone from the cabinet to the floors. Get a few samples, bring them home and see what strikes you. Also, Be sure to get under cabinet LED strip lights. You'll love the look that lighting adds!