UK plugs use brown insulation for the live wire, blue insulation for the neutral wire and green with yellow stripes insulation for the earth wire.
But why this particular combination of colours? The answer is deceptively simple: there is no type of colour blindness that will result in these wires becoming confused.
Above: how a UK plug looks to someone who is red-green colourblind.
Above: how a UK plug looks to someone who is blue-yellow colourblind.
One of the lesser-known safety features of a UK plug is the extra distance that the neutral wire has to travel when compared to the live wire. If someone pulls on the mains cable the live wire will disconnect first, making the plug safer.
Under the IEC 60446 standard only black, brown, red, orange, yellow, green, blue, violet, grey, white, pink and turquoise are acceptable colours for labelling wires. Countries must choose an appropriate selection of colours that eliminates the possibility of confusion.
IEC 60446 colours. From top to bottom: normal vision, deuteranopic vision, tritanopic vision.
During the course of writing this post I realised that there are no copyright-free high-resolution photographs of the inside of a UK plug available online. I hereby release the image below into the public domain.
Click to enlarge.