How this works may also depend on your disembarkation port.
In Los Angeles you don't need to to pre-register to just walk off of Vision of The Seas. You do however have to take your luggage with you and cannot set it out the night before if your want to just walk off.
If you handle your own luggage you can disembark any time after the ship has been cleared, and don't have to wait for your assigned color to be called.
But in large part much of the rules regarding disembarkation will be controlled more by the local CBP officials and not by the cruise line.