Retaining wall, which variants?

... and beyond the formwork construction, that goes as far as sculptures ... (this part was modeled from concrete)

Whereas when dyeing, I would again have the concern that the color does not distribute evenly. With the hollow blocks, I also have the fear that the mortar will fall off over the years...

The even distribution, I believe, is not difficult to achieve because the concrete is mixed in a concrete mixer and the pigments distribute automatically. There are - conversely - also methods for intentionally different coloring.

When coloring, you have to make sure to use cement-compatible pigments. As far as I can see, only "mild" colors are possible because concrete cannot tolerate too much of the pigments ... but you should talk to someone who knows about it anyway ... I have only heard about it and find it exciting.

What I have also seen was concrete whose surface was superficially chipped off afterwards. Also a possibility that can make it (to my taste) "look good."

We still have an old concrete wall - unfortunately not all inherited walls are made of sandstone. As children, we were never allowed to throw snowballs at the wall, the plaster would fall off. That’s exactly what happened. The wall was last plastered in the mid-80s. The plasterer now said that it was like that once. We only had it plastered in 19, so I can’t tell you if that is really the case.

How about greywacke?

How about greywacke?
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To be honest, I'm not a fan of it visually.