You should look at the letter designations in your existing thermostat. Match the wire colors to the terminal letters. A typical furnace and A/C thermostat will have Terminals for R, W, Y, G and C. Most wiring bundles will have most of the colors matching the letters. R=Red, W=White, Y=Yellow, G=Green. But then you have black and orange, so you'll need to make a note of which color goes to which letter. Sounds like you already removed your old thermostat without writing all that down. You can pull the door off your furnace and get the lettering and colors from the furnace end if that is the case. By the way, I have a Trane furnace and A/C system and Trane labels the Common connection as B. So the wire from B on my furnace is connected to the C terminal on the Nest.
