I just bought a new thermostat that is Wi-Fi enabled and it requires connection to a "C" wire. The old thermostat does not have one running to it, but there are other unused wires. The thermostat only controls a hot water baseboard heat system (no fan and no AC). Can I use one of the unused wires for the "C" wire as long as it is connected at the source (boiler) properly?
The "C" wire is typically a common, or the other side of the 24 VAC transformer. The wireless Tstat would need the power source to operate. If there are unused wires in the wiring harness at the thermostat, then using one of these wires for the common is acceptable if that wire is clearly marked (as common). This wire would connect to the other side of the transformer that supplies the 24 VAC (follow the red wire back to the transformer and see how the other side/wire is connected. does it go to ground? does it leave the transformer to somewhere else?).
Yes. As long as the wire is terminated properly at the boiler, the color of the wires insulation makes no difference. In fact, there are no standards that state which color wire should be used for what function, only recommendations.