as close to the tile color as possible, no contrast, for texture, not pattern.

^^

What she said

Epoxy grout to keep it that color. Mapei has one.

Yes as stated and honestly if the space is dark lighting needs an upgrade

if you could lay them in herringbone...I would ...maybe a shade away from white grout. You can use the interest as the floor is monochromatic..cabinets all the one color/ counter very simple.

herringbone....for a step up in detail.... I think youd like it as the cabinets are obviously staying for a while now.....I might actually get a traditional colored tile w pattern but thats not you obviously.

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I would change all the bulbs in your undercabinet lighting from the yellow tones they are right now, to a daylight type, 3500 or 4000k bulbs. It will make a huge difference in how your backsplash looks. If you have yellowish bulbs in the rest of the kitchen then change those too. It really makes a big difference. Personally, I like 4000k bulbs, but, some prefer the 3500k