The wiring technique and the location determine the colour of the wires. A typical two-wire, ungrounded circuit in the United States has a red positive wire and a black negative wire.
The positive wire is red and the negative wire is white in a two-wire system with a negative ground. The positive wire is white and the negative wire is black in a two-wire system with a positive ground. Finally, there are three-wire systems that include a white “centre tap” wire. For these systems, the positive wire is red, and the negative wire is black.