The Princess and Trimline phones all used plug-in transformers and they had to use 4 conductors which was "quad" or "JK" wiring. The red and green were tip and ring, the black and yellow were 10VAC from the transformer.
I believe later on electroluminescent keypads were introduced that did away with the incandescent lamp and the need for a transformer. These used only the tip and ring deriving power from the CO battery voltage.
So no, the yellow was not used after party ringing was done away with as far as I know.
-Hal
