Is there a standard wiring scheme for AC/heater thermostats?
The standard colors can be found at various sites on the web.  Here is the important part from that site (I did the best I could to cleanup the formatting since I don't think we have support for tables here):


Older systems and base signals on newer ones
Code    Color   Signal  Description
C       Black   24 Vac  Common  From one side of the 24Vac transformer (Think of this as 24Vac neutral
R/V     Red     24 Vac  Power to be switched    From other side of the 24Vac transformer (Think of this as 24Vac L1)
Rh/4    Red     24 Vac  Heat call switch power  Same as R, but dedicated to the Heat call switch
Rc      Red     24 Vac  Cooling call switch power   Same as R, but dedicated to the cooling call switch
G       Green   Fan     Fan switch on thermostat - is connected to R when Fan /Auto switch is in the fan position
W       White   Heating call    Is connected to R or Rh when themostat calls for heat. (Might be jumpered to Y on a heat pump. On others might be second stage heating
Y       Yellow  Cooling call    Is connected to R or Rc when themostat calls for cooling. Also Cooling or 1st stage heating on a heat pump. Most often connected to G when Fan switch is set to auto. 

New signals - for heat pumps and staged systems
Code    Color   Signal          Description
Y2      Blue/Or 2nd Cooling     
W2      Varies  2nd Heating     First stage Auxiliary heating on a heat Pump
E       Varies  Emg heat        Disable the heat pump and turn on first stage Aux heating
O       Varies  Reversing valve Energize to cool (Changes from heat to cool on heat pumps)
B       Varies                  Sometimes common side of transformer. Needed on some electronic thermostats or if you have indicator lamps OR   Reversing valve (energize to heat) above. York and Trane sometimes use (B) as common.   Might be heating changeover or on older stuff common of transformer.
X       Varies                  Might be common but sometimes emergency heat relay
X2      Varies                  Second stage heating or indicator lights on some thermostats    Might be emergency heat relay
T       Varies                  Outdoor anticipator reset   Used on GE/Trane/American Standard and some Carrier Products.
L       Varies  Service Light