Telephone line color code changes
As you have found, traditional US household telephone wire contains two pairs of conductors. This was normally used to allow two telephone lines to be brought into the house, each using one pair, with both lines available at all places to ease switching back and forth or to let phones with additional switches (or, later, circuitry) switch between those two lines.

Red/green was usually the primary line, buy it was common for some wall jacks to swap the pairs so a single-blind phone would connect to the other line.

In older buildings, where the phone wiring has been changed repeatedly, the two lines may have gotten cross-connected in ... creative ... ways. Ma Bell was somewhat notorious for sloppy, least-effort solutions.

(Alternatively. rare now, you may also occasionally see the second pair just connected to a transformer. These were used to power the lights in "Princess" phones.)