
http://en.wikipedia.org/wiki/Aircraft_camouflage
Camouflage for aircraft is complicated by the fact that the appearance of the aircraft's background varies widely, depending on the location of the observer, the nature of the background[2] and the aircraft's motion. For this reason, many military aircraft were painted to match the sky when viewed from below, and to either match the ground or break up the aircraft's outline when viewed from above.
비행기의 배면을 하늘색으로 위장하는건 딱히 북한만의 성향도 아닙니다.