Customers have a choice of designs for t-shirts. If they select one, it will be loaded directly into the FPD (moveable). The design is preserved if you change the color of the shirt.
Do I have to create additional products for each color or is it somehow easier?
I hope you understand
You can simply change the color of an element: https://support.fancyproductdesigner.com/support/solutions/articles/5000548185-understanding-the-different-colorization-methods