How a wire is wired and where it is placed affects what colour it is. In the United States, a common two-wire circuit that does not have a ground has a red wire that is positive and a black wire that is negative.
The positive wire in a two-wire system with a negative ground is red, and the negative wire is white. A two-wire system with a positive ground has a white wire for the positive ground and a black wire for the negative ground. Lastly, there are three-wire systems with a white wire that is called a “centre tap.” The red wire is the positive one, and the black wire is the negative one.