no, I think there's enough separation that you can have white shelves by your white fireplace without it seeming troubling that your cabinets are dark -- after all, the trim is white everywhere, and that's usually the color for built-ins. (second choice would be wall color.)

I agree that white would be nice. they should be nice and thick so they look substantial.

I think white would look crisp and classic. As another user mentioned it matches your trim. Good luck with your project! :)

Adding painted cabinetry in your niches will look great, I would suggest Doors at the base and open shelving at the top, leave backless so your wall color comes through, I agree with Cynthia make more monolithic.