In Scribus you can mix pages of different size.
Just pick the isze when creating a new page...
Concerning the colors: in page layout most people have to use a specific set of colors, which they can import or create in Scribus.
"Edit > Colors" lets you create your colors.
In the preferences, you can also pick a different set of color that will be created by default... but i'm not sure, it's a good idea to use bigger sets...