I use MAIL in OSX and do a lot of flagging with different colored flags.
Complication is that I forget what color means filing option I would use.
Turns out you can rename the various mailboxes in FLAGGED.
So my previous “Yellow” is now “Filemaker” with a yellow flag.
"If you can’t make it good, make it 3-D." - the thinking in Hollywood in 2010...