Would suggest you call your installer and have him come look at it. Could be a number of things including improper installation.

The installer has no idea why it happened. All the sheets were installed the same way at the same time. Why some random sheets turned darker and some didn’t we can’t figure out.

That's odd. I'd suggest showing the picture to the tile dealer, but they will probably just blame the installer. Where did you get the tile?

You have a couple of different batches of stone installed. Floor and Decor sourced?

Yes I showed the tile company and of course they blamed the installer. It looks like those darker sheets were more porous stone and absorbed water from the grout which is making them darker. They were sourced from thebuilderdepot.com

Were all the mosaic sheets in sealed boxes and all from the same dye/production lot?

Did the installer apply any type of sealer to the tile?

Is this llne of tile available in lighter and darker versions? That's what it looks like.

Looking closely, the colors within the squares don't vary from tile to tile, there are lots more of the medium tone mosaics mixed in to the point where it is a different dye lot.

They must have got mixed up at the factory and not noticed during install due to such a slight variation. But it could have been caught. Tiles should be laid out and the pattern checked before they are set.

The tile in the niche is not really noticeable.

If you could somehow replace or darken two tiles in the bottom row it would look intentionally random. See my sketch:

Wait...the colors actually changed? I've never heard of that.

Sadly, I think @Minardi is correct. As the old saying goes, "you get what you pay for". I'd bet dollars to donuts this is Chinese made junk where the quality standards are questionable.

This is a forever situation, or you tear out the flooring and niche and re-do it with better quality tile. It's a very small quantity, so the out of pocket for material isn't going to be that much. It's the labor to redo it that will cost you. Lesson learned.

If you want that to look different, then you will need to redo it all. Not "part" of it. Buy better next time.

Tracefloyd- there is no “lighter” or “darker” version. The sheets all looked the same when they were installed and even right after. We have done a number of bathrooms and homes and we always use a reputable professional installer. We absolutely would have noticed during install if the sheets were this different. It wasn’t until the next morning after it was grouted that it appeared this way. Like these sheets were super porous and absorbed water from the grout. All the tile was from the same supplier and is all really beautiful and nice quality. The installer had no complaints. By no means was this tile any cheaper than what local venders offered, I just happened to really love the style/size of tile that I couldn’t find locally.

I think people are just saying that "looking really beautiful" can hide a lot of 'not carefully screened for matching stone source and porosity' which has led to the mismatch. So sorry you are dealing with this.

Buy from a better quality source, and redo it all. You can never fix 2 different shade lots. A good supplier would never have sent 2 shades.

The tile setting itself is superb. Such an even tile line around the perimeter of the shower for instance.

Good labor is just wasted on poor quality materials.

I agree...that's NOT an installer error.

Do you have photos of the tile after it was installed but before it was grouted + immediately after it was grouted? You said that all the sheets looked the same - sometimes you can see subtle differences even though you didn't notice them originally. If you have photos, please post them in a comment.

Also, do you know what type of thinset was used to install the tile + the exact sealer that was used (and when)?

One last important question - do you know the place of origin for the marble? Only marble actually mined in Italy should be labeled as Calacatta (or Carrara). It would be misleading for the retailer to cause the customer to believe that the marble is true Calacatta if it was actually mined in China.