Oh gosh. We all make mistakes. Even pros make mistakes.

You’re going to be pulling that tile out. Theres really nothing you can do to make that work.

The good news is Daltile still makes 4” square tiles in white, they are great quality tiles, inexpensive, you can get them at either big box store, and they make bullnose tile that will finish off your edge.

If you need to take a day and be mad about it, do that after you’ve pulled off the grey tile. You want that off ideally before the mortar has cured. Good luck!

WOW that was big mistake with that wallpaper . Not sure but assume you meant you closed off the kitchen somehow. I need to see the whole space to see exactly what is going on .

If you want uniformity will suggest you remove the gray tile and replace it with the white tile to match your existing.

Redo the whole backsplash. (Hi doggy!)

as a mentioned above re-do, but only go with the white if keeping the wall paper

We need to see the whole space--all four walls.

Here is how to post a Design Dilemma to get the best results. (Post only one thread on same Dilemma. You can add photos to original thread.) You need to show the whole space—all four sides of an entire space—even if it’s about one specific Dilemma. Commenters need to see doorways, windows, closets, finishes throughout the space to make informed suggestions. In open spaces, Commentors need to see adjacent spaces like living, dining, family room areas that adjoin a kitchen. This is so finishes like cabinetry, paint, and flooring harmonize in the whole space.

State right at the beginning what the room will be used for and who will use it. State the non-negotiables at the beginning, what has to be kept. Give some idea of the budget. And provide approximate measurements

https://www.houzz.com/discussions/6417709/unofficial-design-dilemma-boot-camp

Keep in mind that you're inviting some design pros and people with a design eye into your home via photos. If they walked into your actual space, they would take in all of it: all four sides, doorways, floors, windows, ceiling, built-in fixtures, lighting, trim--the "givens"--then the features that can be changed--furnishings, paint, art, rugs, decor items, etc. It's all one first impression. Same thing with photos. That's why it's important to provide well lit photos of the entire space, taken from all sides of a room regardless of the size of your problem.