Fill it with matching grout

How much would you have to purchase? They will often let you buy samples.

Can you go to a tile store that sells this tile and just buy a few tiles from them instead of the manufacturer? We went to one and were able to buy just one tile of one we were interested in to see if it looked good in our kitchen, we only bought one because every tile was the same, your tiles are varied and if I'd gone with something like that I would've bought a handful of them. (By the way, I like your tiles)

I did buy extras but the installer broke them trying to get a straight horizontal cut. The tiles are "art glass" and very brittle. The suggestion to contact a seller directly is a good one. I will probably only need 3 or 4 at most. If that fails, would it be tacky to install some thin white border tiles?

Use a piece of quarter-round or other trim painted a color to blend into the tile.

Would a Schluter piece work? I think trying to match the tiles with another material will lead to disappointment. Definitely try to get more tile but if that's a no-go, the profile will look like a piece of the m/w.

https://www.schluter.com/schluter-us/en_US/Profiles/For-Walls/Decorative/Schluter®-DESIGNLINE/p/DESIGNLINE

Frankly I would get a piece of wood that matches your cabinets and run it along the bottom and make it look like it was always part of the cabinet. Unless you can see this in the kitchen from standing away, no one is going to notice but you.

No one would notice a strip of aluminum or stainless steel here.

Just paint it a corresponding green (or greens). Leave a white line that mimics a grout line.