Have you tried heating them with a hair dryer and see if they turn color?

That would indicate moisture. If the tiles are somewhat translucent could be the gray adhesive they used to adhere the tile.

What is the traffic pattern of the room and does it account for the graying? Contractors always want to pass the buck, but I can see how this might be more of a materials issue than an installation issue.

Will you post some pictures farther back

I think it's that the grout is a lighter color than the tiles and there is glaze color variations in the tiles themselves that show up more against the whiter grout, than it shows up on the ungrouted tile.

Were all the tiles from the same dyelot?

I think it probably is moisture--what is the use of this space? Most tile is installed in places where it either gets wet regularly (shower, backsplash) or periodically (floor that gets mopped, etc.) so it seems unusual that it wouldn't have any contact with moisture. If they've been in for weeks and are just changing color now, it's seems unlikely to be a color effect of the grout or thinset.

If it is moisture and you can leave the space alone for a stretch to let it fully dry out, you could then try sealing the grout to see if that helps.

I would bet if you made yourself a sample board with charcoal grout the tile will look bright white again... (The dots above are the same color)

This is a related post that may hold your answers

https://www.houzz.com/discussions/3579390/why-are-my-shower-tiles-becoming-discolored