If you need to define the color of the document yourself, and you don't feel comfortable doing so, Scribus has an old built-in plugin that can help you. This plugin is called Color Wheel and can be launched from the Extras menu.
The top part of the window lets you choose a base color.
It can be a color you will choose by dragging the red circle around the wheel. The chosen base color is displayed at the center of the wheel.
If you already know how the base color is processed, just go to a CMYK, RGB, or HSV tab and enter the values of each primary or setting.
If you have already added the base color to the document swatch, go to the last tab named Document and select it in the list. The red handle will be automatically placed at the right place on the wheel.
Then, choose one Color Scheme Method. Each scheme will give you different results from two to six colors. The colors are displayed on the right-hand side, in the Result Colors part, and below in the Preview part...