Usually when doing this type of change all the stuff is ripped out all new waterproofing and new tile . Trying to save tile is never a great plan so IMO pick a tile you like for the shower ideally something that works with the floor which we cannot see. Take a pic from further back to show us the whole area .

I suggest you remove all the tile and build your shower properly with the waterproofing material, etc. By not removing the existing tiles you'd have to match them at the bottom where the tub is being removed which will be almost impossible. If the person likes white tiles, simply rebuild a white shower. Consider large tiles for ease of cleaning with less grout.

We are removing the walls of the shower portion as well. Just leaving the old tiled wall for the bathroom part.

Just a few of so many options. If you can post more pictures of the bathroom (all 4 walls) you'd get more precise suggestions.

Chances are good that the 4" white tile is American Olean or Dal Tile, since those are very popular brands and have been for decades. Pick up some samples at your local tile shop, not a big box store, to see which white matches. They have all the trim pieces such as bullnose. Here's a sample of their classic white tiles:

https://www.daltile.com/Search#q=color%20wheel%20classic%20white&numberOfResults=30

Agree with lisedv and patricia. You are better off ripping out everything and replace with new tile.

Just rip out and replace all of it. It will look so much nicer!