The grout is particularly important here because there are already subtle differences between the individual tiles.
A closely matched grout allows those differences to flow together, giving you a softer, more continuous surface. A contrasting grout reinforces every joint and turns the square format into a much stronger graphic grid.
Neither is inherently right. It depends on whether you want the individual tiles to be the feature or the overall wash of colour and light. With the softer shades in particular, a close grout can create a beautiful, almost seamless effect, while a stronger joint makes the installation feel much more structured.
The 13 × 13cm format gives you a quiet canvas, which means the surrounding design can take the collection in very different directions.
Used across a full bathroom, it can create an immersive, softly coloured backdrop. Used more selectively behind a basin, bath or open shelving, the tonal variation becomes a smaller feature.
You can also change the feel through the layout and grout without changing the tile itself. That's one of the strengths of this style: the individual tile is deliberately simple, so the surface, colour and installation choices have room to do the interesting work.