The Mac Mail app defaults to naming the email flags as colors; Red, Orange, Yellow, Green, Blue, Purple, and Gray. Those default flag names aren’t too descriptive, so a much better choice is to rename those mail flags to better accommodate your emailing habits, perhaps naming them as things like “To-Do”, “Family”, “Work”, “Important”, or whatever else. Renaming the email flags in OS X isn’t the most obvious thing in the world however, so let’s quickly walk through how to perform this task.
This works to rename the flags in all modern versions of OS X that support Mail flags, from Lion, Mountain Lion, to Mavericks.
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How to Rename eMail Flags in Mail App for Mac OS X (OS X Daily)