Answer for USA, Canada and countries running a 60 Hz supply service.
Normally a hot wire is black.
When more than two wires are used, a red wire can also be a hot wire.
The white wire is the neutral.
A green or bare wire is the protective ground wire for the safety of the users of appliances and other equipment that gets plugged into the home wiring socket outlets.
Answer for countries in Europe and other world areas running a 50 Hz supply service.
Normally a hot wire is colored brown. (On old wiring circuits a red wire can also be the hot wire in combination with black for neutral.)
For the hot wire going from a switch to whatever appliance is being switched (e.g. from a light switch to a lighting fitting) any other color except green or yellow/green may be used but whenever a brown wire is not used it must always be clearly marked as a potentially live wire by covering the wire's insulation at each of its ends with some standard pvc sleeving that is colored brown.
The color blue is used for the neutral wire. (On old wiring circuits a black wire can also be a neutral wire.)
A green/yellow striped wire (it should never be a bare copper wire) is the protective earth wire (also known as a ground wire) for the safety of the users of appliances and other equipment that gets plugged into the home wiring socket outlets. (On old wiring circuits a green wire can also be the earth wire.)
As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.