Red
Just as in traffic lighting and traffic signs, red indicates a potential hazard in air conditioning wiring. The red wires transmit 24-volt power to both the heating and cooling mechanisms. Do not approach these wires without first deactivating the air conditioning unit and disconnecting it from all power sources.
White
With its high visibility, white is used to mark essential air conditioning wires for operation. White wires are involved in the first-stage heating process where the heating element is initially activated.
Orange
Lighter than red, the color orange marks wires that activate the heat pump and reversing valve in the air conditioning system. While these mechanisms are activated both during cooling and heating, the orange wiring activates only for cooling.
Yellow
Part of the cooling apparatus, the yellow wiring in an air conditioning unit indicates first-stage cooling. This process starts the flow of air coolant as well as activates the compressor and condenser.
Brown
Like orange, brown-colored wiring activates the heat pump and reversing valve of the AC unit. While orange performs this task during cooling, brown activates these mechanisms while heating.
Green
While the air conditioner may cool or heat air, it would not do anything without a fan. Green is one of the wire colors that regulates the operation of the air conditioning fan. Because this fan runs independently of the heating and cooling mechanisms, it can continue running even if the air conditioner is not active.
Blue
The blue wire completes the cooling process, activating the second stage of cooling. This second stage activates a secondary compressor and fan, allowing a greater degree of temperature control.
Gray
The air conditioner fan may also feature gray wiring along with, or instead of, green wiring. This wiring performs the same function as green wiring, activating or deactivating the main air conditioner fan, depending on the signal.
Black
The secondary heating mechanisms are regulated by the black wiring. These wires control a secondary heater that provides additional heat as well as additional temperature control.