We have already seen how to import style and master pages from other existing Scribus documents; importing colors will be very similar.
The simplest method to reuse existing already defined colors is to go to Edit | Colors.
Click on the Import button.
Browse your directories to find the Scribus file that contains the colors you want and select it.
All the colors of this document will be added to your new document swatch.
If you don't need some colors, just select them in the Edit | Colors list and click on the Delete button.
Scribus will ask you which color will replace this deleted color. If this color is unused in your new document, it doesn't matter.
The Edit Colors window provides a simple way to import the colors from another Scribus document: if the colors are already set in it, you just have to choose it. But there are many other ways to do it, especially because colors are considered as frame options and can be imported...