I configured it to green and now it I'm unable to change it back. So does it only work once like that? If I wanted a different color, would I have to buy another?
You can change it back by going in your inventory and removing the tick from the "Active" box. You can also hide the add-on by ticking on the "Hidden" box. When you set it in-active, it'll deactivate and revert back to a normal black user title. If you want a different color, you can buy more color changes, setting each one a different color, and swap them out via ticking/removing the ticks from the "Active" box to the right of each. In this way you can change colors without wasting extra TBT.