This article explains how to select and apply a flag color to an email message in the Mail app on a Mac. It also includes information on how to rename the flags. The information in this article applies to Apple Mail on a Mac running OS X El Capitan (10.11) or later and all versions of macOS.
How to Rename Email Flags in Mac OS X
One way to organize your Mail messages is to use Mail's flags. Marking messages about certain topics with a colored flag makes it easy to keep track of them. Apple Mail has seven colors for email flags: red, orange, yellow, green, blue, purple, and gray. You can mark urgent emails with a red flag, family messages with a blue flag, or work messages with a green flag.
Once you flag a message, Apple Mail adds the flag to the mail header and it appears in a mailbox with the matching color under Flagged in the sidebar. Here's how to flag a message.
Open the Mail app on your Mac.
