there is a new product made by HB Fuller under the TEC brand it's called Power Grout, does not change colors, does stain, nor need sealing, I tried to stain it with a marker and it would not stain. in my opinion it is the best grout made to date worth looking into. good luck on your project

Here is a link that might be useful: power grout

Very pretty. I would do a light gray -- you do not have that many grout lines so you won't see much grout. Laticrete's Silver Shadow is a nice light gray that would blend well with that mosaic.

I second silver shadow. We used that on our shower with 6 x 18 tiles and hex tiles both in white venatino. Looked perfect.