Don't know if that line was originally a color coded one like the distributor/TVS/carb lines or not. My AC wiring and vacuum diagram does not indicate a color code.

#2 - Here's photos of some AC connections including the separate AC-only 30 amp direct fuse line for the blower high speed. I had to replace mine years ago and attempted to duplicate the holder and color code and gauge of the original wire. Note that the connection to the compressor is said to have a diode in series, possibly where that yellow splice is. If it had one, I suspect it was removed for some reason many years ago when I was getting the AC serviced.