‘Red’, ‘Green’ and ‘Yellow’ aren’t particularly meaningful ways to categorise messages, but changing the names to something more useful can make the ‘Flags’ feature a handy tool.
The procedure is quite simple, but the first thing you have to do is actually flag a couple of messages:
4. Type in a new name and press ‘return’ again:
5. Of course, you can change the flag names at any time: