What color mortar was used to set the stone mosaics?

IMO it looks exactly as I would expect. The only relevant photo is the one you are holding up with your hand. The others appear affected by lighting and shadows. Marble is moisture sensitive and will absorb from the mortar & grout. It may lighten slightly after 72hrs. The look of tile after it's grouted always looks different especially mosaics. It's a nice looking splash. Not grouting a mock up prior was your only mistake. Sounds like you did everything else correctly. No there is nothing to be done to change it now.

We used white thinset mortar. It looked a little gray when mixed with water however it didn't have any effect on the marble tile in terms of color. They still looked the same, up until we used the grout.

Perhaps we should have used a light grout, since we find the mosaic is too dark.

What is your opinion on grout haze remover? Our installer didn't do a proper job with removing the grout as he used one bucket and didn't rinse the sponge until many many wipes.

We've tried using a cloth and warm water to clean it and it does look slightly better but the dullness remains.

One other concern we have is that, originally our marble looked polished, yet now it doesn't have the sheen to it at all. Is there any way to repolish it?

Thanks for your help!

Thanks. Unfortunately, I don't think that's
going to be an option at this point. We've read online (countertopspecialty.com for example) that polished marble
doesn't actually absorb that much, so we're very confused about whether the
grout stained the marble tile or if it was something else that occurred.

We used sealer before installation anyway, as recommended by our tile store, however it doesn't seem to have worked as the tile still looks hazy.

So far, some of the solutions we've come up with include, using a magic eraser, using a grout haze remover, using buffing pads/ 3m sanding sponge, or using a hard water / soap scum remover.

I'd greatly appreciate if anyone has any thoughts on this!

Do you have any leftover tile? Before trying these things on your backsplash, it would be good to test to see if they etch or otherwise mess up the marble.

Yes, we have a few leftover pieces we can try these solutions out on. And we have a area behind the stove to test out on the actual tile if need be as well.

The surface of the marble may be polished, but the cut edges of the mosaic pieces are not. The cut edges will absorb moisture into the stone more easily than the polished surface. Sealers, it depends on the type of sealer used, but some sealers can actually slow the drying process.

Like Creative wrote, the installation should lighten a bit as it dries.

For any cleaning, I'd recommend using a pH neutral grout haze remover. Since you used Laticrete grout, I'd recommend you use Laticrete's grout haze remover. Dilution, application, and agitation instructions are on the bottle.

This thread has someone with the same scenario. https://www.houzz.com/discussions/grout-help-dsvw-vd~4687800?n=4

Another reason the whites look cleaner before the grout was put on might be an optical illusion. When you put a white against a very dark area (the spaces between the tiles before grouting) it helps make the white look brighter. Once it's grouted, the light gray next to the white tile doesn't make the white pop as much.

@dostk did you ever resolve this? We had a white and gray honed marble mosaic installed and after grouting, the whole area looks completely different. The stone doesn't look anything like the uninstalled mosaic, now it has almost a dull/frosted glass (not shiny) look to it ... is the best way I can try to describe it. Basically it doesn't even read as marble to me anymore. As far as I know, our contractor's installer did not seal it before hand (it was still in the boxes and packing sleeves) and used a sanded grout. The option of sanded or unsanded was never mentioned to us and as newbies we didn't know to ask. Is there anything that can be done to bring the original look back or is this just expected and we should have done a mock up as someone mentioned before?