SFGate

Ironwood Builders I like the tile the way it looks now! Maybe natural grey? It has a hint of green in it that will work with the green in your glass tiles? The tile suppliers have grout color decks for helping you decide, they are not expensive and come in removable plastic strips so you can put them up against your grout lines and help make a choice.

ASVInteriors I agree, bear in mind that dark grey grout will give you this look - which sharply defines all the joints and going lighter will blend them altogether giving a less delineated look and greater visual continuity (ie space).