I like to see a bit of color in rooms, instead of all the gray, or white that everyone seems to want now. I like the blue thats in your bathroom.

Find a color that you will want to paint on the walls and ceiling. What irritates me about the blue is it makes the room so choppy and stout looking.

I'm all about handsome masculine, which I think will help with the black border, and I'd want to complement the gorgeous greenery out the window. I'd get more contemporary sink and bath faucets and hang a bamboo-ish blind over the toilet so you don't see the outline of the untrimmed window. Add a walnut stained frame around the mirror.

Are these the best? I don't know but they immediately popped to mind without thinking too much about it. Sometimes I just test my gut instincts with a Samplize swatch and see if it is great or terrible.

I need a bit more info as to what is the room next to this like? Usually master baths have some connection to the master bedroom. I have no idea why you choose to keep that death trap tub but I presume budget . IMO just a nice white to match the marble and be done.

You don't mention the color(s) in the master bedroom to blend. Consider painting walls, ceiling and trim the same in a soft white (matte for walls/ceiling and satin for crown/trim) and paint the vanity the French Grey shared above or a complimentary color that goes with your bedroom accents. Same complimentary color in the water closet. A large piece of art for the tub wall and pull from the artwork an accent color for rug and towels, etc.

BM White Dove . The massive tub is overwhelming, I would go with a soft white to neutralize the room and add roman blinds in a pattern like a birds and foliage. A softer black on the vanity cabinet, like a BM French Beret, not a harsh black.

Appreciate all the feedback!

I like the idea of blue but one that is softer with some gray in it. . SW Stardew or BM Beach Glass.

It looks like two different types of marble has been used in the bathroom. It would be great if, at some point in the future, you could remove the huge tub + surround of Carrara marble (it's a very busy Carrara marble also).

The marble in the shower is nicer - I'd definitely keep that! I believe that it's Arabescato Corchia:

A slab of Arabescato Corchia:

It also could be Calacatta Corchia:

Here are some colors that work well with Carrara and some Calacatta and Arabescato -

SW Sea Salt is very popular bathroom/bedroom color - it looks very nice with Carrara marble:

Here are some colors that work with SW Sea Salt - this is from Etsy (= you have to pay to receive names of paint colors). I think Sea Salt is on the wall (sometimes it looks more green and sometimes a bit more blue). My guess would be SW Rainwashed on the vanity.

This photo below has a vanity painted same color below that is on the above walls -

You may want to use a wood stained vanity in order to add some warmth to the bathroom.

My primary bath is white and gray and I used a pale green as the wall color (BM Woodland White at 50%), with darker green towels. This was the concept from the beginning, so I like it. In the off-white category BM Classic Gray and Paper White come to mind. You have gotten some good suggestions, so go off in the direction that appeals to you and start sampling. You will have to experiment to see what you like and what goes with your type of marble.