I know the NEC doesn't do color codes but is there an accepted (traditional) color code for HVAC thermostat wiring?

+24V=
0V=
Heat=
Cool=
Fan=
Etc.=

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.

Well, let's see ...
The red seems to always be the 'power in.'


Saturday - I've edited the daylights out of my post, because it was simply ... wrong.

I've always seen:

24V common: Red
Heat: White
Cool: Yellow
Fan: Green
Blue: Second heat stage
Orange: LED for aux. heat
Brown: LED for emerg. heat

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.

I could add a lot more, but start with these

As klein man pointed out, there's common practice but no real standard so it unfortunately varies from brand to brand. Google is your friend, and will answer your questions for your particular unit