In the US of A, it's supposed to be black.
For wiring in the USA the Neutral conductor is required to be white or gray by the National Electrical Code.
No, you cannot wire a 120 volt ballast to a 347 volt circuit. The ballast is designed to operate at a specific voltage, in this case 120 volts, and connecting it to a higher voltage circuit like 347 volts can damage the ballast and pose a safety hazard. It is important to match the voltage rating of the ballast to the circuit it will be connected to.
black
AWG # 10 wire on 30 amp circuit.
You can't. The 120 volt GFCI is probably just a 2-wire (hot, neutral and ground) You would have to run a new 3-wire (2 hots, neutral and ground). The two hots are how you get the 240 volts (120+120=240). Also you must make sure the wire is gauged properly. #10 wire for 30 amps, #12 wire for 20 amps, etc.
No, a GFCI outlet is designed for use with standard 120-volt circuits. Attempting to use a GFCI outlet with a 240-volt split circuit could cause damage to the outlet, the circuit, and could present a safety hazard. It is not recommended to use a GFCI outlet in this manner.
Because the white wire on a 120 volt circuit is the neutral wire that is connected to the silver screw on outlets and switches. It is connected to the neutral bar in the service panel.
No, you cannot simply cap one leg of a 240-volt circuit and change the breaker to convert it into a 120-volt outlet. This requires rewiring the circuit to provide 120 volts, including changing the outlet and possibly other components to be compatible with the lower voltage. It's recommended to consult with a licensed electrician to properly convert the outlet.
Yes, but it will not work.
A 240-volt circuit typically consists of two hot wires and a ground wire, with no neutral wire. The hot wires each carry 120 volts, while the ground wire is used for safety purposes. In a 240-volt circuit, the hot wires complete a loop by connecting to a load or device that requires the higher voltage to operate.
14 AWG wire normally supplies 15 amps. Watts = Volts x Amps x Power Factor. Power Factor has a maximum value of one. If you have a 120 volt circuit that would be 15 x 120 = 1800 watts or 1.8 KW. For a 240 volt circuit 3.6 KW.
Yes but it is not compliant with the NEC you can use a 240 volt circuit, (Ground, Hot, Hot) you would use a (either) hot wire with a ground wire and that will be a 120 volt circuit. I strongly advise you not to do this though because a ground wire is not trully meant to carry current and it will not be approved by any inspector.