Build a terraced end house with an additional unit (GÜ) on your own

So we have chosen differently for all 3 bathrooms (G-WC fancy, Master bathroom elegant+timeless, children's bathroom "normal") - but in the end it's totally a matter of taste! You will surely also find opinions from people who have done everything the same. I think there are just too many beautiful tiles for that.

Bathroom on the upper floor and attic floor the same color (gray/white) or two separate tile colors?

Whether in the garage, whether the tenth floor, who cares about the architecture?
In high-rise buildings, I find it practical if the floors have different colors - then you can see faster if the floor to get off at has already come. But with the bathroom, you don't need that. The color has no informational value and can be chosen purely according to taste. I'll take the gray and my brother the beige.

Grey-white reminds me of the 90s
To your question: I would also keep it different. Go ahead and have some fun with it.

We basically have the same tiles in both bathrooms but with different "highlights" (borders, etc.), so the bathrooms do not look completely the same.

Understandable - it's a matter of taste. The tile lady thought it would be nicer to have a consistent color concept throughout the house. And since we're completely covering the floors in natural oak (Parador modular one), fully gray/white bathrooms would fit.

Although the tiles with mosaic are from the same series - just that some are gray and the others beige.

But how is it with these stainless steel tile strips? Is that purely an aesthetic thing or do they also offer an added benefit? At about 35 meters needed and €17.40 net/m, that adds up quickly.

Only appearance. Aluminium is cheaper, but also looks different. Ask to see samples.