I'd use white or light gray.

You need the grout color to help unify the tiles better (the main tile is reading warm toned whereas the accent tile reads cool/blue). So for that reason the accent tile needs a warm toned grout (cream or warm white) If you use gray there is the risk that it will only make the mismatch more obvious rather than less.

I like alabaster.

I just wanted to give everyone an update. Turns out they didn't have Alabaster OR Cape gray. I went with Oyster gray, and it was perfect.