That’s a live firing exercise!
The following video was filmed in mid August 2015, at Seungjin Training Field, South Korea, during 2015 Integrated Live Fire Exercise.
It shows, among the others, ROKAF (Republic Of Korea Air Force) F-15Ks and KF-16s, dropping bombs on the range, ROKA AH-64, MD500 and KUH-1 helicopters, MRLS as well as some U.S. Air Force A-10s using its GAU-8 Avenger 30 mm hydraulically driven seven-barrel Gatling-type.
The drills were also supported by a South Korean Boeing 737 AEW&C Peace Eye that can be seen at the beginning of the video releasing flares.
The interesting footage shows an air power demo (to flex muscles against North Korea and China) rather than actual firing training: needless to say, no combat plane would ever use purple or yellow smoke during a real combat sortie.
Many thanks to @andiegewehre for the heads-up