Wiring – Can a green wire be used as hot if it’s reidentified

Absolutely do not leave the wires alone wrapped in electrical tape. Electrical tape does not have enough longevity to safely protect the exposed leads and can create a hazard in the future.

The blue wire exists to support the future addition of a light. The polite thing to do for your future self or future owner of your home would be to connect the blue and red wires. However, it is not necessary; if you do not want to, you can cap both leads with wire nuts.

Note that it is also legitimate to connect the black on the fan to the red from the ceiling, and the blue on the fan to the black from the ceiling. Assuming you have two switches, that will swap which switch controls the fan and which switch controls the light.

If the existing black-black connection doesn't work, you can try the black-red. If that does work, it would suggest that the switch for the fan has failed, although it could also indicate other wiring problems.

If the old fixture worked, at least one of the white wires is hot.

The easiest way to check which is hot is to use a non-contact tester. If the fixture is already removed, leave the two wires bundled together with a wire nut over the end. Put a wire nut on the single white wire. Turn the breaker back on. Only one of those should register on tester. That is the hot side. The other should not register. That is the neutral.

Once you know which wire is hot, mark it with black tape or a permanent marker.

If the the fixture is still attached, you can do the same thing with the fixture hanging down. Once you know what is what, turn the breaker back off before wiring.

The black wire goes to the hot wire, the white to the neutral. The ground wire should be attached to a green or bare wire if there is one in the box, or to the box itself if it is metal. If not, just cap it.