I would look at a grey if the wall tiles are white. We used a colour called Limed White and it's fabulous, a real stone-grey with a hint of warmth in the hue.

Your mockup scheme is similar to our master bathroom we just completed - photo below. I know you mentioned storage being a concern - our mirrored cabinet has offered an amazing amount of storage in a very small space and also helps bounce light around. I was worried about black taps etc too so we kept chrome fixtures but added in a matte black candle and black potted orchid to create a similar effect which we can change out later on. Our floor tiles are dark but because they are semi-polished they actually bounce around a lot of light and don't make the room feel dark if that is a concern. We also have a huge fiddle leaf fig in the corner (not pictured) which adds a really nice feel to the space - a plant like that could help bring in the outside greenery in your bathroom. Instead of the storage tower you have to the left of the vanity could you do say some floating wood shelving, possibly with baskets added for more hidden storage? You could also bring in the 60s theme with what you displayed on there eg. photos as you mentioned? In regards to the window - a previous house we lived in had a window at about shoulder height in the shower - as well as letting in heaps of light it was amazing for storing toiletries etc!

I suspect you have lovely hard wood frames and studs underneath. I'd strip back right to those, they'll give you a great base for resheeting. More importantly it will give you a better chance to make everything square and even for tiling. And yes, while you are there strip back above the picture rail.

how old is the house as you may need to check for asbestos on walls and floor and is teh whole house a slab on ground if not the floor under the tiles may not be concrete..., when replacing floor tiles i would also replace skirts to ensure waterproofing is above the floor all around (and avoid a potential leak at the join) and depending on what you are planning to do (ëg install new shower stall or wet floor?) i would also consult your tiler about "levelling the floor" which is much more complicated than we think.. and take your time removing the old tiles, especially on the walls as it's better to do as little damage (to the structure and yourself) as possible and you will find many online DIY instructions for retiling that are worth reading before you start...good luck with your project