Sometimes the English do something that makes so much sense you have to wonder where the idea came from. If you ever had the opportunity to look under the hood or dash of your British car you would have seen, among other thing, lots of wires, the newer the car, the more wires. All these wires are different colors; some are multi-colored, and all for a very good reason.
Sometime before WW II (I've been unable to find out exactly when) the British instituted a wire color coding system for all the cars manufactured in their country. It started out very simply to identify the basic electrical systems and has grown, as the cars became more complex.
A black colored wire indicates a ground circuit wire, the wire connecting an electrical unit to ground, usually the car's metal chassis. A brown wire is HOT or one that always has power and is not fused. These are the big wires that carry battery power to and from the fuse block, voltage regulator and generator. There are also smaller brown wires that run the wiper motor park circuit.
This is where the extra complexity comes in. Since the brown wires run different circuits it was necessary to differentiate the wires for each. The TRACER is a different colored stripe running the length of the wire to indicate the particular job of that wire. Therefore a brown wire with a yellow tracer is for the generator warning light while brown with a white tracer is for the ammeter.
As a quick note, the colors are abbreviated in the various manuals. Some are obvious like R means red, Y means yellow, G is green. Since black is denoted by B, N was used for brown and U for Blue. So a brown wire with a white tracer (power to the ammeter) would be NW, brown with a light green tracer is NLG (the windscreen wiper park switch).
Blue wires are for the headlights with plain blue being power to the dimmer switch, while the power from the switch is denoted two ways. Blue/red (UR) is for the low beams and blue/white (UW) is for the high beams and the indicator lamp.
The use of tracer colors is especially evident with the white wires. White denotes a circuit that is powered when the ignition is on. A plain white wire runs the fuel pump, ignition relay, and various fusebox connections. White with red (WR) is the power to the starter solenoid, and white with green (WG) is power to the radio. White with black (WB) is power to the ignition coil unless there is a ballast resistor then the wire is white with light green (WLG).
On the early cars with few electrical accessories green was the color for fused power from the ignition for such items as brake lights (GO or GP) and the fuel gauge (GB). When turn signals were added Green was used here also, green/white (GW) for right turns and green/red (GR) for left turns.
As customers demanded more conveniences the wiring grew in complexity. Now, along with green, light green is also used as the base color for various applications like screen washer pumps (LGB) and hazard warning lights; light green with brown (LGN) is the color here since the hazard light system needs an always hot circuit to operate without the key being turned on.
For the next extra color they couldn't use grey since G was already for green, so S for slate was the obvious (?) choice. Slate indicates circuits that are hot when the ignition is off, such as emission control power. Purple is for always hot circuits with fuses such as courtesy lamps (PW) or key buzzers (PG or PK (K for pink).
So, you see, there is a method to the madness. If you know the system you can even sort out a wiring harness that you never saw before, with no wiring diagram, for a car that someone else dismantled 20 years ago!