During the attack on Pearl Harbor on December 7th 1941 lipstick on the forehead of the patients told the triage story

 an F on the forehead of a patient meant this person was going to die no matter what. F for Fatal.

an M on the forehead of the patient meant that they needed Morphine a pain killer. M for Morphine.

Various other letters on the forehead meant other things then.

You didn't want the patient to know what was on their forehead because the knowledge might kill them unnecessarily or cause them unnecessary distress which might not be good.

This is how they kept those alive that could be kept alive on December 7th 1941 in Honolulu, Hawaii.