I would just paint. I don't think you need anything. it's a bathtub with a separate shower unless you plan on splashing around. I would also paint trim around window and baseboard white. it really stands out right now. I would rather want people to notice beautiful shower and tub before trim..

Why didn’t the builder (or you, if you were getting the supplies) get the tile when the floor tile was purchased. Did the builder misunderstand your plan?
Agree that you don’t need tile. It looks fine as it is. If you want a more decorated look, your river rock/nature theme could be carried to the walls surrounding the tub by getting some of the birch tree wallpaper that is so easy to find. Check it out on Wayfair, there are many manufacturers and so many colors that you could find one that goes well with your wall color. This would work best if the wall behind the person is empty, no vanity or linen cab or such.

If you are using black as an accent color elsewhere besides the trim (towels, picture frames, cabs, hardware) then I don’t mind it as trim.

There has to be a different brand of tile that is close in color to what you have now. You have to go searching for it.

OR, at the chance of sounding not up to date, and don't know if you would even like it, use the pebble tile as a wide, maybe 4 feet, accent strip around the tub. Two feet above the top edge of the tub and two feet below, to protect from direct splashes. Don't know if it would look good or if you like that idea, but trying to be creative here. Maybe you can somehow put the tile or paper that looks similar on the wall and see how you like it first.

that is the smallest niche I've ever seen. I hope you don't need more than trial sizes of one shampoo and one conditioner.

why not forgo the tile and have them do a vinyl wainscot (I assume the inspection needs something on the wall to pass) ?

since they screwed up, they will do this free of change, yes?

Long story short, we picked the tile over a year ago and I don’t know how or why the builder didn’t order enough. I called the company to verify myself they no longer make it and have yet to spend the hours to call all the places they supply to to see if they just happen to have a box in their showroom. Yes, the builder is willing to make it right and is giving us the chance to find tile or something that we’d be happy with around the tub to protect the wall. We do have two kids that will occasionally want to use the “nice” tub, so I’m sure there’s going to be some water that splashes out then. The pictures of the wainscoting

Wainscoting is really starting to grow on me. Then we don’t have the stress of hoping the color of the tile looks like it belongs versus it just being an oops. I don’t know how the C river rock would look just being on the wall and not creating a design with it on the floor. And the builder will not do floor work to make this right.

Yes the shower niche is laughable. Of course we could have done it different for extra money, of course. So I’ve already found an over the glass hanging unit on Etsy that looks great and gives the farmhouse feel to it.

Wainscoting will look purposeful here - tile that doesn't quite match never will. The river rock would look ridiculous.