Is it possible to display category list as main page?
Yes, I think this is possible using an individuell template.
Is it possible to create new groups and change color of username based on group color?
Is it possible to create categories that only selected groups can access?
Not in general. It is possible to restrict the access of categories to admin/mods.
applied-geodesy.org - OpenSource Least-Squares Adjustment Software for Geodetic Sciences