Before radios came into standard use, they used various flare colors (or combinations) for signalling.
(Additional because of phone interruption)
In WWI, the RFC had issued a standard for flare signals:
Red flare meant "Rally on me";
Fired by the flight leader, it was used to form up the flight on his position after taking off or to reassemble the flight after a fight.
Fired by a flight member, meant they are having trouble and need the flight to rally to them for help.
White flare meant "Return to the Aerodrome";
Fired by the flight leader only, over enemy territory, the flight was to immediately form on the leader to return to base. If fired over friendly territory, they were to return to base at best possible speed without formation.
Green flare meant "I must return to the Aerodrome";
Fired by the flight leader only, the flight command was transferred to the leader's wingman.
Also, an interesting situation where flares were used:
During the Battle of Malta, a Ju87 became seriously damaged and tried to make an emergency landing at Lucca. The British AA was fierce, preventing the Stuka from approaching, so the Stuka's pilot fired three red flares followed by a green flare.
The British at the airfield saw this and replied with a white flare, at which point the AA ceased and the Stuka was able to make thier landing.