This?
http://customer.honeywell.com/techlit/pdf/PackedLit/69-0780.pdf
Look at page 2 where it says thermostat.
Red = one side of transformer, 24 volts AC
White = call for heat
Yellow = cooling
Green = run fan all the time
Black or the common terminal, the other side of the transformer, not typically run to the thermostat. You need to find out what they are doing with it.