We have a house in Spain with marble floors and have this issue in several rooms- we noticed the main areas after about 6 months but have had new areas emerge even 15 years later. We’ve had several specialists to look at it over the years and had explanation of oxidising, damp coming through, being laid on sand, sand walked on them, sea air, wrong cleaning materials ...We’ve had the floors specialist cleaned, ground and polished many times and it’s still there. I know some neighbours just replaced them with a less porous stone. Could you ask the marble supplier as I think you do need to avoid it pre installation not rely on being able to clean it afterwards.

Does anyone know where I can find black and white rectangular tiles (like the ones pictured), but much smaller (mosaic or up to 30cm wide)? I have attached a picture of the style I am looking for in white. Any help will be greatly appreciated!

I decided to do what your thinking about (coloured grout on floor, then white grout on walls). I think it looks OK not odd, but obviously is personal opinion. Pic attached part way through work, I didn't use a grey marble but a gold vein marble. I think I would have preferred tiles lined up between floor and wall still but you don't notice too much after a bit. Gives you an idea on effect

I can only see your 1st pic. I like the walls and doors etc but find it too much with the throw. They’ve used different colours for curtains which I’d repeat for accessories. It will obviously make the room feel bigger, taller and it’s very calming. Our bedroom, walls and all wood, is one colour with the exception of the windows (venetian so wanted to highlight) the fireplace and the ceiling. Alternatively it can depend on the features or lack of ie what you want to hide or highlight. My kitchen was colour drenched years ago but because it’s a very ”bitty” kitchen, built in dresser, corner cupboard, wall units, the list goes on! Double drenching has been appearing of late too, I’ve added a link re the photo issue! Maybe once Houzz is working upload some more pics? https://www.littlegreene.eu/double-drenching#:~:text=In%20'Double%20Drenching'%20schemes%2C%20combine%20colours%20that%20have%20different,from%20across%20the%20colour%20cards.