Question

the wires inside a cable each have there own plastic insulation .why does the insulasion have diffrent colours

Answer 1

A cable is a bundle of two wires or three wires or a thousand wires,
inside a single 'tube'.  In order for the cable to be useful, you need to
be able to match each wire at one end of the cable to the same wire
at the other end.  If each wire has a different physical appearance,
then it's easy.