Is the Platinum the Laticrete Platinum? If so what about Silver Shadow? It is a very soft gray that is almost white, but looks great with Platinum. Are you using cementitious grout or epoxy? I know many have used SS for their light or white tiles.

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.
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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:
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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.
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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.
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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.

Wow, your bathroom is going to be really striking. May I ask where you got the tile in the herringbone pattern?

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 don't think it is a good idea to submerge tile in sealant because it is cautioned to avoid sealant in the grout line as the grout wont stick. There are others who have used SS with their marble and it looks good. Bill V. has posted pictures of SS with marble. Do a search for Silver Shadow on the Bathroom forum and see if some post pop up with some pictures.

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.

interesting about the grout not sticking to sealant. i'll have to talk to my sealant guy. thanks.

I just went back to look at your old posts on your bathroom and found your grout discussion. Near the bottom Bill V. shows some bathroom tiles that he laid with the Silver Shadow by Laticrete. I see that the Platinum you refered to on that post is something different than the Platinum that Laticrete makes. I used Laticrete on my slate floor and the straight Platinum is a darkish gray with slight blue undertones as the tile stick sample indicates. The bluish color is hard to see unless it is next to another color like a pinkish gray grout sample.

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.

Here is a link that might be useful: Your grout post.

Thanks.
So how do we prevent the gray grout staining the edges of our white tile for our foyer and hallway floors? Does one type of grout work better for that?
Lastly, we will have 1/8" grout lines, but the tiles have some chipped edges (they are antique) so there will be areas where a grout live is not even in thickness. Might a sanded be better then?
Thanks!

I don't think the cementitious grouts cause the halo staining, just the epoxy. Have you looked into urethane type grouts? They are supposed to be durable too. Go to the john bridge forum and see what you can find regarding epoxy and staining light marble if you don't get this question answered here. Or post a question specific for staining light marble and grouts.