I am retrofitting a Lutron Caseta dimmer switch to an existing light fixture in an older home. The fixture has line, neutral and ground, but the wire that was run to the switch box does not have a bare ground conductor. It does however possess a third, red wire that is unused on both ends. The Lutron Caseta needs to be grounded (since the fixture is Low Voltage). Can I connect the red wire to ground so the switch is grounded?
Not unless you wrap the entire length in green electrical tape, or you wired the circuit with 4 AWG or larger conductors. National Electrical Code says that equipment grounding conductors can be bare, covered, or insulated. It goes on to say "Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes..." (250.119).
If you wired the circuit with 4 AWG or larger conductors, then you can reidentify the red as ground. You just have to permanently identify the conductor at each end, and at every point where the conductor is accessible.