While there is no regulated standard for color-coding HVAC wires, industry practice has the G wire connecting the thermostat to the fan. This wire is typically green. The R wire, typically red, is for power. Some systems have separate power wires for heating and cooling and are labeled RH and RC respectively. The typically white W wire is for auxiliary heat; i.e., a second source of heat. The Y wire, which is typically yellow, connects the thermostat to your air conditioner. Finally, the C or “common” wire is used to carry power and is typically blue (think cerulean if you need a mnemonic).