Identification friend or foe
From Halopedia, the Halo wiki
Identification Friend or Foe is a system used to tag friendly or hostile units for easy identification on a Heads-Up-Display or motion tracker. Friendly units appear as yellow dots on the moption tracker, while the crosshairs of a HUD turn green when pointed at them. Hostile units show up as red on a motion tracker, and the HUD crosshairs turn red.
