Smoke-producing flares are used for a variety and diversity of reasons by the UNSC. Different reasons appear to have different colors of smoke attached to them.
Signal Flare: "Smokers" are used to alert allies of a friendly position. They utilize thick, green or white smoke.
Smoke Canister: These are used to signal for pickup after a mission, or a request for an equipment drop, or even a need for emergency evacuation. They create thick, blue or red smoke. These are also referred to as "smoke" grenades.