In the United States military, the various colours of smoke created by smoke grenades do not have preassigned meanings or uses. The specific meaning of a given colour is determined by the needs of the user at the time of use. This means that the same colour of smoke could mean different things in different situations.
Coloured smoke grenades were first used by American forces during the Vietnam War. They were used to prevent the Viet Cong from employing a tactic known as a false flag manoeuvre. The Viet Cong would attempt to draw enemy air forces into an ambush by popping standard smoke grenades, or "false flags." By using coloured grenades, American troops could provide air forces with a colour-specific target and help them to avoid enemy ambushes.
Military grade smoke grenades come in a variety of colours. They can be red, orange, yellow, green, blue, violet, black, grey or white. A military grade smoke grenade is basically a simple steel cylinder filled with a coloured smoke composition. Its colour is indicated by a strip of paint around the top of the cylinder. Emission holes in the top and bottom allow the smoke to escape after the grenade has been triggered.
Having several smoke colours available makes it possible for ground and air troops to coordinate multiple operations at the same time. All the ground troops have to do is pop two different smoke colours and make sure their air support knows what each colour signifies. For example, they might pop green smoke to indicate wounded soldiers on the ground and red smoke to mark an enemy target.
Since there is no set meaning for any colour, every colour of smoke grenade can mean whatever its users want it to. Some potential uses include marking the location of wounded soldiers, providing a target for an air strike and marking the location of friendly troops in need of extraction. They are often used to provide a smoke screen to mask troop movements and attacks.
Coloured smoke grenades are not limited to military uses. Smoke grenades are also used routinely in paintball events and airsoft-gun matches, yet these rarely meet military-grade standards. Nonmilitary smoke "grenades" are usually high-volume smoke candles or smoke bombs, which are significantly less expensive than their grenade counterparts. In either paintball or air-soft matches, different smoke colours can be assigned different meanings just as they are in real combat settings.