keyboard shortcut for different flags

There is a built-in shortcut for the red flag - command-option-L.

There are probably 2 ways you could do it for the other flags.

1) You could create custom keyboard shortcuts for them by going to System Preferences->Keyboard->Shortcuts and going to the App Shortcuts area, then adding a section for Mail (use the + sign, and select mail). You'd add a shortcut for "Orange" and all the rest of the colors. You can use this trick to define menu shortcuts for any app. Here's one I just did for "Orange"

2) If you show the toolbar, and drag the flags into it, then each item in the toolbar gets a keyboard shortcut (command-1 to show, control-command-1 to file to that folder). That should do it.

My recommendation is #1, as it's so flexible. I used it to define keyboard shortcuts for the Notes app, e.g., to automatically create a bulleted or dashed list.

Glad to help. This one is always a neat trick; it's possible to do a ton with custom shortcuts but it's kinda buried.