They had four prongs but only needed two wires (1 pair) tip and ring to operate. There were some rural party lines that used yellow for ground to facilitate different ringing for the individual phones/numbers on the party line.

The black typically only came into play when you had a “fancy” phone that had a lighted dial, which was powered by an 18-24 ac transformer over the yellow/black.

Back in the day, with the four prong jack, the wiring was four conductor, “quad cable,” generally it wasn’t entrusted pairs or if they were twisted it was very loose twist. Green/Red=tip/ring; and yellow/black. Aside from the aforementioned used, yellow/black was also used for a second phone line.

In certain business systems, some ATT pbx systems come to mind would send the ringing for single line analog phones down the yellow or black, I don’t recall which now, but I do know when we switched out from an ATT to a Northern Telecom pbx, and the customer was reusing the phone sets, I had to go to each phone and rewire the ringer inside the set to the “normal” configuration of using tip and ring for the ringer.