Rootes used a lot of Lucas electrical products on their vehicles and the wires you quote are from Lucas standard colour range.
If I recall correctly , purple and black is for horn circuit and usually found on indicator/horn switch going off to horn.
In Lucas system main wiring colour is linked to purpose of the circuit and tracer colour helps by being more specific to component e.g. Blue = headlamps, blue/red = dip beam and blue/white = main beam,white for ignition fed circuits - If you look at wiring diagrams in Rootes Manuals you will see letters alongside each cable and then a key at side of diagram saying what colour each letter means e.g. R Red W white Y yellow etc
Hopefully you will find a supplier that sells old Lucas stuff[IIRC Ripaults supplied cable to Lucas in 50 yards or metre reels] or you can salvage some of cable from another wiring loom