You can delete colors assigned to your shop like this:
"Catalog > Colors > [desired color] > Options > Delete"
Please note that you have to select the motifs under "Catalogue > Designs" I have to adapt. If you use the color you want to delete here, it may cause some problems. in your system, because you deleted the color afterwards.
If you have taken motifs from the "System Motifs", please check them. even after removing a color, since these build on the standard colors, that are initially assigned to the system.