I'd use a 24x24 porcelain tile in a dark charcoal, black or even white.

Have you looked at Tilebar? go to floor tiles.

it would look like this, but in whatever color your choose

I really like this one. It's the Tilebar Cavatina in Bronze.

and the Cavatina Corda

https://www.tilebar.com/collection/tile-mosaic-collections/cavatina.html

this is a limestone look porcelain.

and they ahve these cool hex tiles that look like cement. one of 10 diffcolor combos

https://www.tilebar.com/pergola-wood-graphite-12-5-hex-matte-porcelain-tile.html

https://www.tilebar.com/texstone-gris-9-matte-porcelain-hexagon-tile.html

Or, I love this look w/your walnut and your sage green. It's a hone Gray Basalt. (seal first before grouting)

These will both last a lifetime and never go out of style.

Black slate

Marmoleum would be my choice. First off, it's fabulous. Secondly, there are many brown-flavored swirly patterns that can complement a wood floor really well. (The colors that resemble cork are good with wood floors. Also, consider actual cork!) Or you can do an actual COLOR Marmoleum...

Can't make color recs based on the photo (and you'll want your cabinet color picked out as well).

Marmoleum requires a really smooth subfloor and you might find that after the tile demo required for it, you have more height than you realized and can then put in hardwood. But If the kitchen is staying open to the walnut, I wouldn't want a different wood in there.

Beth, thank you for the detailed recommendations. I've been living on tilebar this week - we're also planing a bathroon. And Fori, marmoleum is definitely on the list.

I am trying to be bold with color without pushing it to extremes. I really need to get a profesional in here...but I thought I'd get as far on my own as I can,

Why not just get unfinished walnut and have it stained to match IMO a much nicer look than that floor being different.

We used glue down cork stained white. Love how easy it is on my legs and back. It goes well with wood floors.

We just had Marmoleum installed in our laundry room and bath that coordinates nicely with our new "Provincial" hardwood. It is called "Fruit Punch" and it looks fabulous!

Beth, the walnut is beveled and hand scraped. Plus it’s 3/4 inch and the existing tile is about 1/4 inch. It would take heroic measures to match it.

A friend of mine suggested replacing all the flooring with luxury vinyl or porcelain, but I can’t bear to junk real solid wood floors. And we’d have empty out all our stuff.