The Irish rules are a little more limiting:
(quote from the ETCI (ElectroTechnical Council of Ireland)
The following limitations apply:
1) Green/yellow cores shall be used only as protective conductors and shall not be converted e.g. by sleeving for use as neutral or line/phase conductors.
2) Blue cores shall be used only as neutral conductors and shall not be converted e.g. by sleeving for use as protective conductors or line/phase conductors.
3) Brown, black or grey cores shall be used only as line/ phase conductors, and shall not be converted e.g. by sleeving for use either as protective conductors or as neutral conductors.
4) Where more than one conductor in a multi-core cable is fed from the same phase, both shall be identified by the relevant phase colour, using sleeving as needed.
5) For single-core cables used for three-phase circuits, the core colours shall be the same as for multi-core cables.
6) Where single-core cables are used to supply single-phase circuits fed from a three-phase distribution board, the respective line/phase conductor colour may be extended to the single-phase circuits fed from that phase.
7) Sleeving or tape used for identification purposes shall be permanent, heat resistant, colour-fast, non-conductive, and shall comply with EN 60454 or other appropriate standard.
8) Four-core cable without a blue core e.g. NYM-J shall be used only for circuits with balanced three-phase loads e.g. three-phase motors.
(Also the phase order is Brown (L1) Black (L2) Grey (L3)) Single phase systems, just like flexible single phase cables on appliences, use Brown as the live.
