Each user can be in a number of different groups, but only the one that is their "default" group will affect the coloring of their username.
From the ACP, Users & Groups, Manage Users, find the user, "Groups" in the drop down form. You will be able to see which is set to default for that user.
Or if you're also marked as the leader of the particular group, you can change the settings from your UCP. UCP, Usergroups, Manage Groups, click "Members" next to the appropriate group. You can use the checkbox and submit form to make that group the default for the selected members.