The North Korean system of insignia largely repeats the Soviet one.
Judging by analogy, then:
Colored shoulder straps are shoulder straps of everyday uniforms.
Reds with a black fringing are infantry.
Blue with a red edging - there was no such analogue in the USSR. I can assume that these are either shoulder straps of the Air Force or shoulder straps of internal troops (an analogue of the NKVD).
Field shoulder straps (used in combat) have the main field green (or khaki). Edging on field shoulder straps:
The infantry is red;
Technical units (engineers, sappers, builders) - black.
Do all green shoulder straps with a red edging - you will not be mistaken.
For officers in the USSR, the edging of field shoulder straps could differ from the soldier's one. Perhaps this was the case with the Koreans.