When you take friendly fire in Halo 5 the damage indicator in the HUD is Blue/Green. This is a departure from previous games where all damage was shown in red.
Some examples:
Shot in the back in campaign (Outside Xbox)
Shot by teammate (Podtacular)
Team grenade (Kinda Funny Plays)
Halo 5 Guardians Gamescom 2015 arena b roll footage.