Always follow the thermostat manufacturers instructions whenever changing the thermostat. Always turn the power off at the air handler and the condenser and make sure there is no voltage at the transformer before proceeding. The colors below are the most likely colors used for most installations however your thermostat wire colors may be different than what is indicated here. There is no official standard for thermostat wire colors. Thermostat wiring colors are not standardized but these are the most wiring methods used based on my years of experience in the HVAC field where I successfully wired a lot of thermostats. For instructions on how to wire a thermostat please see our how to wire a thermostat page.