The Set Mail Status action works on a selected message in Apple Mail. 

Note that the currently selected mail message is not necessarily the front window, it may be a message selected in the viewer behind the front window, especially if the front window is a reply or new message.

Select which status to set using the //Set Mail// pop-up:
  *flag index.
  *flagged status.
  *read status.
  *junk status.

Set the status using the //to// pop-up:
  *off — represented by the value 0.
  *on — represented by the value 1.
  *value.

Note: if you select //value// then //off// and //on// will no longer be available.

If you choose //value// then you must enter a number or choose one from the stepper control.

Note that in Apple Mail flags may be:
  *Red.
  *Orange.
  *Yellow.
  *Green.
  *Blue.
  *Purple.
  *Gray.