The standard wire colors for a home thermostat in North America are black, red, blue, white, and green, with some wires appearing bare. The colors are tied to specific functions for the wire to make them easy to identify for those working on them.
Black and red wires are known as hot wires. In a three-phase system, a blue wire is also a hot wire. White wires are considered neutral. Wires that are either green or bare are ground wires. It is important to note that non-professionals adjusting the wiring of their thermostats run the risk of both personal injury and property damage.