The red wire is usually positive in electrical circuits. Although red usually denotes a positive circuit, there is nothing about the wire itself that makes it positive or negative.
Positive and negative terminals only occur in direct current (DC) circuits. Electricity results from the flow of electrons. In DC circuits, one end is always positive, while the other end is always negative because electrons always flow in the same direction, creating a positive and negative terminal. By contrast, in an alternating current (AC) circuit, the direction of electron flow changes periodically, so no part of the terminal is always positive or negative.