I guess in WW1 there were no international signs, all kinds of signals were used at the time were strictly national, and defined for certain squadrons or even single missions.
In WW II e.g. three red flares being fired were used internationally in emergency, as a sign for "don't shoot, will land here in your territory". Used by english and german planes, towards each other and to ground fire positions.
(see e.g. Stuka landings in Malta, or generally damaged planes going down in enemy territory in WW2).
Reconnaissance planes will always automatically be able to observe a green flare at the aerodrome, when beginning a server mission (like field is cleared, mostly with a text "the xyz begins its mission")
Two yellow flares indicate a balloon having sighted an enemy plane in the vicinity.
Squadrons in RoF have their own secret code for shooting flares, but you dont really need it with this feckin' teamspeak (which i consider as cheating).
I sometimes shoot a green flare at a damaged landing opponent, indicating i will not shoot him. There's no "code" though.