Here is Silver Shadow epoxy on the bottom vertical grout spaces. The top area was grouted with a 50/50 ratio of Platinum and SS. The middle row was with Raven and SS. Note that there is a halo around the marble, like it is still wet. Epoxy can do that with marble, I just looked at my tile boards after over a month of drying and the halo is still slightly there with the darker grouts. But not as marked as in this picture. So that is something to consider.

Here is the Silver Shadow epoxy with my bright white tiles. With these tiles the SS looks light gray, but with the tiles above it looks almost too white for my taste:

I took Laticrete epoxy Platinum and mixed it with a little bit of Silver Shadow for an in between gray for my black slate floors. It is in a 3:1 ratio. The colors mixed very nicely. The grout sample on the left is the SS and the one on the right is the Platinum. It didn't take much SS to lighten it up significantly. If I did this over I would just have used Platinum.

My DH really likes the combo. He thinks it ties in with the tub. It is relative isn't it, because it is not white at all but appears white.

I will be having a marble listello, as seen above in the sample board, with my white tile wainscot. I might mask the marble off when I grout the white field tile with Silver Shadow. Then I would plan to come back and grout the marble with a reverse blend of Silver Shadow 3parts to Platinum 1 part. Nothing like trying to complicate things. That's one of the benefits with DIY. But if I was paying someone to grout, I would not consider these mixes.

I think the marble on my bathroom floor absorbed the gray grout, too. There is a halo around each tile. We are going to submerge our foyer's white tiles in sealant to prevent this, hopefully.

Maybe silver shadow will look too white with my walls then?

I personally kind of like the halo effects:)

My floor tile was a 12x12 special order Brazilian Black Montauk slate, that I cut into widths for 6x12. Not actually 6x12, I had to account for the grout width in my cuts, so they are less than 6x12. Thanks to Mongo for helping me figure that out. It was special order from Home Depot and was perfectly gauged and absolutely no flaking.

Regarding the sealant, I have been told that the sealant is applied to the set tile, and carefully to avoid getting it into the grout lines. The sealant helps with grout cleanup as it keeps the grout from sticking as hard to the surface. I have only grouted twice and both times I sealed the surface. Both times the grout cleaned up very nicely - both the cementitious and the epoxy types.