We're going to die, aren't we?

Don (husband) is trying to hook up a new thermostat. This involves killing the power to the old thermostat. He's not sure which is which, cirsuit wise.

I am calling my dad. I know enough about power to know not to screw with it =) I have been zapped once fairly well in the past six months (during a network upgrade, at work, don't ask), I don't need another zapping.