What do the colors of thermostat wires mean?

Thermostat wiring color codes are available at Dr-Fix-It.com. Each color has a special function. The five principal wiring colors on thermostats are red, green, yellow, black and white.

Additional thermostat wiring colors include orange, blue and gray. The red wire stands for 24-volt heating and cooling. The yellow wire is for first stage cooling, and the second stage cooling wire is blue. The gray or green wire is for the indoor fan. The white wire signifies first stage heat, while the black wire shows second stage heat. An orange wire most often represents the reversing valve or the heat pump.