YOu could add a small glass accent band and put a 4x4 white under it. Or add just a row of black or gray glass subway vs a ceramic tile.

If you can, replace just the bottom section with white subway tiles. That will give you the cleanest look. Even if you have to cut the subway tiles it will be less noticable than trying to introduce another color, shape or type of tile to the limited area of the backsplash.

You probably will not get an exact match to the existing subway tile. I would try something such as DirectBuy has suggested if you can match the whites and the sheen of the tile.