I don't know what it is, but every thermostat I've wired has little letters next to the screws for the black, yellow, green, red wires. If that's cast right into the plastic, it must be following some color code standard somewhere.
Here, Im an HVAC Controls Tech. This is a complete diagram for different systems. Ive added Gas, Electric furnaces and heat pumps. I can add water source HP's if desired.
(R) Red - Power (W1) White- Heat (W2) Blue- Second Stage Heat (Y1) Yellow First State Compressor (Y2) Jumpered (Board Controls Second Stage Compressor) (O) Orange- Reversing Valve (Heat Pump Only) (B or X) Depeneding on Manufacture will be Common (Heat Pump Only) (Brown and Pink) Outdoor Stat (Heat Pump Only) (G) Green - Blower Relay
Aux Heat for heat pumps no longer is hooked up, the fuel master does it for the unit.