Wrong terrace slabs were delivered but have already been installed

Such a huge difference, but you could have noticed it during delivery as well. What does the specialty store say about it?

The fact that you have already installed it is not the supplier's problem. Otherwise, you can demand a correct delivery. Why the supplier made a mistake is not your problem.

Gut feeling says that you should have checked the correctness before installation, since you ordered the panels. Dismantling and reinstallation would therefore be your responsibility. Return delivery and new delivery would be the supplier's issue. It would then still be questionable how to assess the depreciation of the installed goods. If nothing speaks dramatically against the color tone, a price reduction is certainly the better option for everyone.