Two questions:

1) why two showers? wouldn't the smaller one serve better as, say, a linen closet?

2) what are your other finishes, beyond the plain tile and dark vanity? because the answer might involve all of that. Especially floor tile.

Anyway, I hear that it's possible to remove a row of tile, but there is a risk of breaking adjacent tiles, so not every contractor will attempt it. If you can find one that will, then you can get an accent and paint that play together -- maybe something like this.

Also note the possibility of just adding accent at the edge, or maybe above the vanity...

Here's some paint ideas-

Pearl Gray 863 Paint · More Info

Seapearl OC-19 Paint · More Info