Signal flare

The signal flare is a common tool used in the Military by the Stationary Guard and the Scouting Legion. The signal flare has a variety of different uses all depending on the mission.

Uses
While the Signal Flare as many uses it main purpose is communication over long distances. The Stationary Guard has used it as a way to inform there higher ranking officers how there mission was going. While the Scouting Legion used the signal flare as a way to to detect and avoid titans.

Ammo
While it all depends on the mission at hand the uses for the ammo here are from the 57th Expedition beyond the walls.

Red - Titan spotted

Black - Deviant type titan spotted

Purple - An emergency of some kind

Blue - An order to retreat

Green - This lets everyone know that the commander has seen the previous flare launched and shots a round in the direction that the squad must adjust to.