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

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.