While telephone and radio had become widespread by WWI, different colored fireworks were also used to send coded messages.
While the telephone was extensively employed for communication purposes, absolute reliance was not placed on it, and the troops were profusely equipped with numerous methods of night signaling. The code was changed from day to day, and great attention was paid to drilling the men in the use of pyrotechnic signals. The chief advantage lay in the rapidity of sending and receiving. There was no carrying of messages: there was no ambiguity of language, and there was no “listening in” on the part of the enemy.
An example in battle: signaling to your troops an imminent gas attack using green fireworks.
For instance, on some special night, green might be the signal for gas. When the advanced positions detected gas, a green light was shot up from the Véry pistol, this signal was relayed from the trenches with V.B. cartridges, and eventually a rocket ascended high into the heavens, expelling at the height of its trajectory a little green light suspended from a paper parachute. More detailed information eventually found its way over the telephone communication. A similar signal the next night might call for the barrage.
Hey, if it works, it works. The newest tech isn’t always the best way to communicate.
Skyrockets and Flares as Aids to Our Fighters: Uncle Sam Had to Learn How to Make Fireworks When He Got Into the War, Because Telephones and Wireless Were Inadequate for Communication at the Front
Published: Sunday, August 17, 1919