When you you create a PDF from Scribus there are, as far as I understand it, four possibilities:
1. Export to PDF/X-3 will not convert any colors at all, but will instead embed profiles (not sure of printer or screen/web has any effect here)
2. Export "for printer" will create a CMYK PDF
3. Export "for screen/web" will create an RGB PDF
4. Export "grayscale" will create a grayscale PDF