Bummer... If it were me and I didn't have the funds to redo the entire bathroom tile. I would look for one of the dark blue or medium blue tiles and see if I could find other tiles imperceptibly close and tile the area that needs tiles with those blue ones.

I would go to your local floor n decor, home depot, lowes, tile supply and borrow or take samples of every blue tile you can find to try to find one that matches. If you can't, you might have to bite the bullet and retile the bathroom. You don't want a patch work job on your beautiful bathroom.

You deserve a nice place to bathe.

Although you want to avoid it, re-tiling the whole shower is the only way to make it waterproof. Re-tiling is vastly less expensive than repairing water damage later.

For new tiles, look at Daltile and American Olean. They make several shades of white in several sizes with all the trim pieces.

That patchwork kludge is another leak waiting to happen. Redo the whole if you want it done correctly.

I'd go for the largest format tile to avoid more seams.

Fit them so they look intentional, as if to make a new pattern.

You could do another row of colored ones and then try an off white tile with a line of separation it might not be as noticeable

I think you could use the tile you have on your floor if they are suitable material for shower....it's a choice that doesn't add an additional color to your room. If you are able, redo the entire shower area but sounds like you already said that's not in the budget. As an intermediate, can you at least remove another row of tile so your shower tile color transition is at the same height as your color tile row in your tub area? I can't see well enough to tell where the next row of tile in your tub area ends.