I like the look of a simple backsplash using the countertop material rather than using a mosaic there. We used a 6" tall splash instead of 4". We just finished our master bath last fall and we used the Emser Lucente Lazzaro for one shower wall and the niches. Lazzaro has a lot of light greenish-gray-marble colors in it. Lucente comes in a lot of colors. My local showroom only had 2 colors on display, but the Emser website shows a lot more options. If you use Lucente I have one suggestion regarding installation. I don't know if this will occur with a different color, but with the Lazzaro color, there were some marble pieces that had black dots in them. That was fine as long as these dots occurred randomly, but it just so happened that some of them lined up in one area when being installed. The installer was so close to it that he didn't realize. His helper was standing further back and noticed. While things were still wet, he cut out the offending pieces with a utility knife and replaced them with a piece from another sheet. Just an installation detail I thought I'd mention. Our colors were: Emser Uptown Sugar Hill for field tile in 12 x 24" (it's a light gray/concrete color with some variation), Lucente Lazzaro linear mosaic for one wall and niches, QDI The Rock 3" x 3" floor tile (charcoal gray), marble countertop and tub surround with marble splashes, white vanity cabinet and tub skirt panels, a large linen cabinet stained in a medium brown tone and some floating shelves also stained, and existing wood floors. We thought a white and gray color plan might be too cold so we incorporated the greenish-gray mosaic and the stained cabinet, plus a teak bench in shower to bring in some warm elements.

I’ve seen black islands with super light gray cabinets for all others and think it looks amazing. Gray is so easy and a fresh clean slate. One thing to consider is warm vs cool colors. When we were building our house I kept picking cool grays. I realized that everything we own is warm colors. I pivoted a little in colors to a warmer gray - we have knotty alder cabinets which are warm tones, and managed to find a flooring that was light enough but still warm. Gray is not bad. Just factor in the rest of your house too! Beautiful kitchen - floor plan looks awesome, updated colors will be spectacular. You do have space between ceiling and cabinets - have you considered adding the cabinet to go the ceiling that is glass front with a light in it? I always love that look - adds brightness but also warm & cozy somehow. I always love the way it looks with white or light gray cabinets too!

Waht are the floors now? IMO wall to wall carpet is never a good choice for flooring so I would do the same floor as the main floor then add as colorful a rug as you like in eacj space . BTW wall color is usually the last thing you choose after all the other elements are in place and barn doors are not a good idea for bathrooms or bedrooms I still lke wall color to be a bit more neutral and then add all the color you like with bedding , rugs etc easy to change and the walls just act as a backdrop for all the wonderful color. See how all the colors pop in this space

Are you sure that the abutting built-ins can't be removed? They look like after thoughts. The crown molding looks so overdone. I would think it would match the crown in the rest of the room. Wish you could remove them both...but: Paint all the color of you wood work.