Cannot add a new color to my colorpalette.

I extracted a color palette from an image in Coolors. Imported it in Blender (4.2.0), in object mode, as a Grease pencil object. Renamed it, all following the tutorial exactly, but when i 'extracted palette from color attribute' only 5 from the 8 colors where created. I don't know what I am doing wrong, since i am not able to manually add those other 3 colors to my newly created color palette. I am not able to create even another color palette, by color picking or choosing on the color wheel, since all newly added colors show up as black.