You can't change your name color whenver you like. Established group is a normal member group(members that post more than 100 posts are in that group), you're a Colonial(a higher ranking member group, approved or you might calls it), which is represents by the brown color, "overwrites" it, this also makes it easier to recognize between Colonials and other member groups. If you're not a Colonial, your name would have been the green (the Established group's color).
Though you can still get blue name (Paragon's group) or pink name (Hotliners), by reaching a certain amount of posts that group required. And please, don't spam just to get it lol.