I don't really believe the Federation is colonialist. I think they are very freedom conscious and respect other cultures. But as long as the dates stand, it looks like the Federation simply claimed a planet that was already settled on. And sometimes some of their behavior is suspiciously colonist at times. Setting up hidden labs and studying non fed cultures; turning over non fed systems that belong to cultures over to other powers; things like that. The "wicked" admiral was also acting like a colonialist in certain ways.