thank you...I'll get it eventually 
When you start Scribus, you'll see a "New document" dialog. Instead of creating a new document by pressing "OK", or opening an existing one, press "Cancel", so that Scribus is running, but with no document actually open. If you go to
Edit > Colors now, as utnik suggested, you'll notice that you can choose between different color sets.
If you create a new document after you have chosen a color set of your liking, you'll be able to use colors from that set in every new document you create.
But if you want those colors to be available in another, already existing document, first save this new document containing your chosen color set (name it, e.g.:
Example.sla). After that, open the document you want to also contain those colors from
Example.sla and import them using
Edit > Colors > Import > Example.sla feature.