Would paint a pale blue gray. It would go nicely with your cabinets. Add fabric to to windows and some trim around windows. Nice space

I like the painting, but the wall looks a little top heavy. Paint or tile inside the space behind the fireplace box. It needs to be black so it looks like it takes up that whole space. If you want to do a mantle, just put in a simple white shelf (like a floating shelf) that goes from one bookcase to the other. If you put in a mantle, you can continue the dark color out to fill in the space under the mantle, or add some molding under the mantle on each side of the fireplace & paint it white like in the 2nd picture.

The back splash is certainly taste specific. If you are going to remodel in the spring, I certainly wouldn't spend a lot though. I have never done this myself, but I am pretty sure you can paint the tiles. That would be a huge improvement. Here is a blog that addresses that project. Hers looks great. http://www.11magnolialane.com/2013/03/12/painted-tile-backsplash-in-my-kitchen-a-year-later/

I love the tall units but you need to go straight to the ceiling to prevent dust and dirt settling on the top. I would put a row of small up and over cupboards along the very top as an option. Very handy for unsightly bulky items. I don't like the end open shelves by sink very dated but what you could do is a long double shelf all the way along the island with a wine racking either side for balance vertically. I think shelving each side of the sink would be better. You have a lot of cupboards but no deep drawers which are highly valued these days. Good selling point. I Like your colour suggestions... The little Greene paint company have a lovely grey so does farrow and ball. Where are you sourcing your Quartz and what flooring? Do your ovens have easy open shelf doors ..handy as they retract so don't get in the way as you take food out..Neff offer best ones. If you want a better balance of tall you could always do fridge freezer one side and pull out pantry on the other with racking. Then on sink side don't go all the way along the wall just have it as centred space so you can pull out larder doors. On your island you will need your bin as this'll be prep area? What is the room opposite tall cupboards? Need the island to look good from that side as that is what you will see. Hope of help. I'm embarking on the same thing and have spent hours and hours researching!