Presuming they used white mortar it very well will return to its original color. Do you have a new tile you could lay on top and rephotograph?

Did you purchase the marble - or did the contrctor supply it?

What type of marble is this? Is it from Italy or China (place of origin - not place where manufactured/cut).

Marble is translucent - and may take on quite a bit of water when being set (and again a little when grouted). Not all pieces will take on a lot of water - even if bought in the same lot. Some pieces will be less porous than others.

I had Carrara marble installed in my bathroom - and many pieces were darker - some significantly. Within a few days, they had mostly lightened - within a week they were basically back to the original white color. My tiles were 3x9 (shower walls) and 4x12 (around stand alone tub). Marble LFT (large format tiles) will take longer to dry. I have read that marble LFT may take a few weeks to dry (from some pros)

HOWEVER, marble doesn’t typically turn brown and then whiten back up - marble will look much darker (gray) - and then lighten once it dries.

Your photos look like there is some rusty color on it - which makes me concerned that the marble had iron deposit in it - and the water has caused the rust color.

Post a photo what the original tile looked like as suggested above by Hallett & Co + add the type of marble + place of origin (was it mined in Italy - like genuine Carrara and Calacatta are - or if was it mined in China, etc.)

Marble is porous and absorbs moisture during the setting process. Yes, it lightens up as it cures, over a matter of weeks.

If the coloring is orange, it could be iron deposits in the marble as mentioned above. That doesn’t resolve on its own. I had this happen with my 2 inch marble hex; the problem was resolved by replacing all the orangey tiles with other white marble tiles. FWIW I had purchased tile from a very reputable local tile store; the tile was described/titled as Carrera, but after this problem occurred I contacted the tile company, and it turned out the provenance was China, not Italy. Both the person who sold me the tile and the person at the tile company who looked up the information were surprised, as was I.

Op has left

Dani, I was very upset indeed! It does seem like false advertising to name a marble tile “Carrara” when it is from China, grrr. This bathroom floor was part of a big renovation, and happily I had no other significant issues with any of the other tile used; all were from different manufacturers. It’s been about 18 months since the iron-stained tiles were replaced during installation, and there have been no further problems.