Sounds like your daughter's already got some interior designer instincts ;) Loving the black feature wall, and you don't want to take anything away from such a strong statement, so I think you should go with white... or at least a very light grey, e.g. Dulux Lexicon quarter (on the left)...

I work as a colourist and the easiest method for a upmarket look is to choose an exact colour to match one of the colours in the tile. I would take the tile to a paint shop and get the colourist it match exactly one of the greys in the tile. Make the depth of the colour a light version. All white walls are on the way out. Lappatto tiles usually have some texture as they are semi polished. This is the latest look in tiles as they give a shine without the maintenance of polished porcelain which had been in. They are usually less slippery than satin or polished tiles depending on the brand.

What a great job you did. So good that you just went with white. The concrete floor tile on wall behind the vanity would have been too busy after now having seen the finished product.

I did that in one of my bathrooms but the feature tile had more pattern (warm Grey brown streaks) so looked softer than the all black against the white walls