A tip, if you allow me: when you go to Edit > Colors, and click on New button, notice a dropdown list in upper right corner which contains a lot of color sets. Using, e.g., "Scribus X11 Grey" might be helpful as starting point for creating your desired grey (even if creating grey shouldn't be hard: set C, M and Y values to zero, and change value of K only; but for creating other colors this feature might be of great help).
Note: I assume you're using Scribus 1.4.0. I'm not sure about 1.3.3.x series.