It happens to the best of us. We come up with a great system for keeping ourselves organized, and then poof, we forget one part of the system. Say, for example, that you used orange to color code all personal messages. Then, suppose you don't get a personal message for a while, and you forget what orange means. No problem. The decoder is a just a click away.
Note You can also right-click the color next to a message to see the name of the category associated with that color.