The older station wire was just red/green/yellow with the 3d wire used for selective ringing in party line service.
I have a Western Electric 500 set dated 1962 and that has just a 3-conductor red/green/yellow cord fitted for such use.
I also have a Princess phone with the lamp wired to yellow/black. The lamp feed actually goes via a series resistor which is shunted by a pair of hookswitch contacts so that the lamp runs runs dimly when the phone is on-hook (there is also a cut-off switch in series with the resistor to turn off the "night light" feature).
As I understand it the original power units employed for these phones had relatively high-impedance secondaries to limit the current in the event of a short or cross with the red/green C.O. pair.
