Not sure what your asking but standard HVAC LV wiring colors are simple
Red, transformer power to the thermostat, if it is a split system with a separate heat system such as a boiler that don't have a common transformer, you will have two reds (power supply) to the thermostat, Rh and Rc, h for heat, and c for cooling
White is for heat
Green is for fan and is used in cooling or manual fan on.
yellow or blue is for condenser in cooling.
common from the transformer is run to all three condenser contactor, gas valve or control for gas valve, and 24 volt fan over ride relay.
In heat mode fan is brought on by a fire box temperature switch in the line side of the fan control that also has the high temp limit that kills the power to the transformer line side.
Get into zoneing and its a whole another world, but the basic's are the same.