Jim Whimpey brought up a complaint about flags in OS X Mail:

Why does Mail have these much too subtle flags instead of full colour labels like the rest of the OS!?

Jim Whimpey via Twitter

I figured I’d tackle the problem because I imagine other users have the same issue. I’ve made some OS X services to assign flags with matching message backgrounds. These services just contain a few lines of AppleScript that change the flag and the background color of the selected messages. If you want to take a look, you can open them in Automator. Here’s how you use it.

1. Quit Mail if it’s running.
2. Download Full Color Flags
3. Extract the contents to ~/Library/Services/
   
4. Optional: Assign keyboard shortcuts to the Services.
   

To use these, open Mail, select a message or messages, then go to Mail > Services and choose your desired flag.

And this is the result:

This solution works for me. There is no guarantee that it will work for you. I am not liable if it does not work for you or if it damages your computer or data. Proceed at your own risk! You may use this solution free of charge for any purpose.