White.

Use the same color grout you're using on the rest of the shower tile, and I'd also consider removing the metal frame and just use grout.

Do Not use white grout. Even with plain white tile, light gray grout is best.

Do as calidesign says: same grout thruout and no metal framing, just grout.

Why did you repost???/

Because it seemed from comments that I had not been clear in what I had asked