The basic thermostat wire colors are red, green and yellow, according to DoItYourself. There are usually five wires in common hot water systems and up to eight varying wires in heat pump thermostats.
The red wire connects to terminals that start with the letter R, including R, RH and RC units, depending on the purpose of thermostat, states DoItYourself. These terminals enable connection between the electricity supply and system circuitry. The yellow wire is typically used in air conditioning units. It is attached to the compressor and connects to the Y terminal. The green wire is attached to the G terminal and controls fan relays which trigger the blower fan. Programming the thermostat to reach a particular temperature triggers the system. The G terminal receives power once the specified temperature is attained, causing the blower to start.
Some systems also use other colors that are not commonly found in all thermostats, explains DoItYourself. The white wire is attached to the W terminal and activates the heating. The terminal is directly linked to the furnace or boiler. The black wire is usually linked to the C terminal, which manages the transformer. It is often used in electronic thermostats that require only 24 volts. Dark blue wires are connected to the B terminal in heat pumps.