Thermostat cables typically have multiple wires that are used for low voltage purpose and have several colors. The following is a guideline but may be different in your system The white wire is what connects to the (auxiliary) heat on your system. The yellow wire connects to your compressor. The green wire connects to the fan. The orange wire connects to your heat pump (if you have one). There can be two separate red wires. One is for cool (RC) and the other is for heat (RH). The blue is the common or â€œCâ€ wire. There may also be 2 black wires, these are used for various heating systems for outdoor applications.