I used the silicone grout that was matched to my grout. I used Laticrete products.

Thanks, enduring. No shops around here seem to stock Laticrete silicone, even the ones that sell the grout, etc.

I ordered mine online from a link on the Laticrete site. They were FAST and packed very well. here is the link if you care to go that route.

Here is a link that might be useful: tile store online

I've had good results with the premium GE silicone that's readily available - can't remember the name but their better of the two main varieties you'll see.

.....though I usually use DAP 230 latex caulk because it's easier to apply (or at least least easier to clean up after).