When your Armatar turns to the starry night shield, it's because you had a, let's just say 3000 AP Armatar. Then, some of your comments got deleted, and you lost that AP, so it automatically put you with the starry night shield.
@DeathTheKid1313: It's just a common glitch, nothing to worry about.
When AG added those "color shields", the people that had the original blank shield Armatar (which was the original default shield), were given the starry night shield.
Yes it is. You select an armatar at a certain level say at 1000 AP and you have 1000 AP. Then your AP goes down to say, 985 AP. You lose that armatar because you don't have enough AP. Then it goes to the starry shield.
That's not unlocking it, though. That's losing points from unnecessary posts/comments or if someone clears their comments and you lose your current armatar.
My armatar changed to the starry shield several days ago a few times throughout the day. However, that was when they were doing a maintenance thing. It's nothing to worry about.