A wire through which an electrical current circulates must be made of a conducting material (its atoms must have outer electrons which can easily be separated from the atom and which travel through the material). These materials are metals (copper, aluminium and other metallic alloys).

The characteristic magnitudes of wires are: resistance, which opposes the flow of electric current, and width, which depends on the electrical network, as it must not become too hot and melt (or set something on fire). These two magnitudes are related.

In addition to the alternating current cables that reach houses, the telephone is connected to a source of direct current with a low voltage and intensity, which transmits signals by using the change in intensity caused by the microphone. Since the end of the twentieth century, there are signals transmitted by electromagnetic waves (television signals, for example) which are detected by antennae and are amplified and transmitted along coaxial cables, protected by a metallic shield. Nowadays, there are also singals transmitted through the Internet by means of computers, which use small voltages and intensities with an order of magnitude of a few milliamperes.

Cables that carry information (and not just current) must be protected with metallic shields so that the data transmitted is not perturbed by external signals.
The colour of the plastic jacket does not affect the conduction of electricity, it is just used by the electrician to know which cable is connected to a high voltage, which is connected to ground, etc. just by looking at the end of the cables. The ground cable is usually yellow and green and it usually goes from the electrical sockets in a building to a metal spike buried in the foundation. If part of the current goes through the case of an electrical appliance because of a contact problem in the wires, most of the current will leave through the ground cable instead of going through a person's body.