Does it have TBA on it? Your cabin number would need to be filled in along with your name, address, etc. Usually the colored side only indicates which deck you are on and whether it's forward or aft. The rest of the information you fill out on the flip side.
The color does indicate where you are staying on the ship and it is for the porter's to be able to organize everything easier, for instance they can know what area to take your luggage by the color of the tag. Also when you debark the ship, it keeps helps keep things in an orderly manner by grouping people as they get off the ship according to luggage tag color and organizing the luggage terminal by tag color.
The room number should be in your documents and you need to fill out your information on the tags as far as your name, room number, phone number, flight information, etc.
But if he has a GTY cabin, the cabin number will not be on the luggage tags. The TBA (gty) tags are a gray-blue color, and do not indicate an area of the ship or a cabin number. They will be exchanged at the pier for the correct color-coded tag, and the cabin number will be hand-written on it.
You will get your cabin # at the port. We booked a GTY and were upgraded from 6A GTY to 8A GTY by our TA, then we were upgraded at the port to 8B(which is just a deck higher) most upgrades are within the class(outside to higher outside/balcony to higher balcony) from what I understand.
If you have your name on your luggage don't waste your time filling out name and address on the TBA tags, the porter will exchange them anyway.