Needing Help with new thermostat install (C Wire)
It's a little unclear exactly what you're asking, but it sounds like you're on the right track.  If the orange wire is unused (unconnected at both ends), you can use it as a C wire.  Connect the one end to the thermostat's C terminal. Then connect the other end to the C terminal in the furnace.

If the furnace does not have a C terminal, please see this answer for help figuring out where to connect the wire.

According to the schematic, the grey wire should be C.