A player will receive a yellow skull if they attack someone marked with a black, red, white or revenge skull (orange). The attacker is yellow skulled only for his victim; this is done because even player killers have the right to defend themselves against other players that might try to kill them. Killing someone with a yellow skull will be a justified kill.
Yellow Skulls are special. If a player who has a black, red ,white or revenge skull is attacked by an unmarked player, the attacker will get a yellow skull which is visible only to the player he attacked. This allows even a black skull to kill in self-defense without receiving extra penalties.