Under the new identification colours for fixed wiring introduced by amendment two to BS 7671, black should now be used in new installations to identify a phase rather than a neutral conductor.

For all single-phase applications it is preferable to use cables with brown (L), blue (N) and green and yellow (E) cores. However, where three-core cables marked in the new three-phase colours are used for single-phase applications, appropriate over-marking at terminations must be used. It is proposed that a black core is used for the protective conductor, brown for the phase conductor and grey for neutral.

This breaks the association between black and neutral. The black core will be over-marked green and yellow and the grey core over-marked blue.

Source

Electrical and Mechanical Contractor