Replacing old thermostat with new wi-fi Honeywell
You can't simply connect any extra wire to the C terminal on the thermostat.  You have to also connect the other end of that wire to the C terminal in the furnace. 

If you include a photo of the wiring at the thermostat,  and a photo of the wiring in the furnace I can give you a detailed list of instructions for hooking it up.