Now to answer your question, it means that the negative wire is the common wire in an electrical circuit. The colour codes have changed today, but live wires go from brown, white, yellow, red, ect. negative are today always blue or should be and earth is always green, but always use a tester top make sure.Source(s):ow to answer your question, it means that the negative wire is the common wire in an electrical circuit. The colour codes have changed today, but live wires go from brown, white, yellow, red, ect. negative are today always blue or should be and earth is always green, but always use a tester top make sure. Source(s):