[Oldcodger]

Dave ,as you say ,there's convention. Internal cables are either 6w or 8 wire. A six wire cable had Blue + orange + green pairs. Pair one in any internal cable is always Blue. Pair two is orange and pair three is Green. I always used 8 wire with an additional brown pair as I worked on a lot of systems which had the need for 2 pair ( 4 wires ) per phone. Internal colour code is Blue, then orang,then green then brown .
However , consistency is KEY. As in a properly wired extension system ,the speech path is on the blue pair on terminals 2 & 5 of a socket. Ringing passes from terminal 2 to terminal 3 via a capacitor (to next socket). With a phone plugged in, ringing current passes from the capacitor in the master socket , to terminal 3 of the master- to terminal 3 of the slave , then via th "bell" circuit of the slave to terminal 5. Th purists can harp on about the need for certain colours being on certain terminals, but at th end of the day , consistency is KEY.