We have seen a similar issue in the past. The problem occurred when the porcelain tile on the floor did not have 100% contact with the tile adhesive, this caused water to seep down though the grout join and hold in pockets under the tile in the areas where there was no direct contact. The water being trapped in the pockets under the tile does not evaporate and can leave the tiles looking stained. If this is the case with your floor tiles the bottom row of carrara marble tiles are simply acting like a sponge drawing water up. To fix the problem we would suggest that the bottom row of carrara marble and the porcelain tiles on the shower floor need to be removed and relayed ensuring full adhesive coverage. Unfortunately this may compromise your under tile heating and waterproof membrane. Proceed with caution.

Hi Sue, I think it would look amazing if you used some tiles with a bit of texture. The wood look tiles that are popular at the moment would be an example. If you chose a colour that toned in with your floor you would get a really nice cohesive look but with a bit of interest. I've attached a photo of a recent design of mine as an example. Just a suggestion!

I see what you mean, however marble is a classic material that wont easily date if you select a quality tile. What you accessorize the tile with, like tapware is more likely to date but is more easily replaceable. I don't see black tapware going anywhere soon however stay away from copper

Hi Doug, looks great! regarding the floor my only suggestion would be to avoid the grout line down the centre as you have it. it is the first thing that draws your eye, and if you are going to the detail and expense you are then looking down is not what is going to show it off. From that I mean make it complementary to other things not competing.... eg a simple 1/3rd 2/3rd off set to break up the line and allow your ceiling feature to draw the eye. cheers