To the ECM I don't know what color it would be. That's something I haven't had to do yet. Trace that far back I mean. I would think you would have to run into the steering column writing before the ECM.
On another note the tail lamps on YJ's are just generic trailer lights. Wiring colors will be different on the tail lamp side. All the wiring color should , "should" be the same on the jeep main harness side. The only way I figured out that blue with red tracer was that wire was broken at the connector you took pics of. And also at the right front marker light.
"I may not change the world, but I'll damn sure leave a scar"