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
A similar limitation is found in BS7671 -- Green/yellow cores may not be re-identified for any other purpose than earthing.
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
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
However, BS7671 contains neither of these limitations.
Basically, you may not re-identify a green/yellow wire to use it for something other than an earth, but you may freely re-identify any other color if you wish.
So a blue on switch loop is sleeved brown to use as a return. If you are using 3-core (e.g. SWA) which has the brown/black/gray phase colors, you can leave the brown as is to use as the phase, then sleeve the other two conductors in blue and green/yellow for neutral and earth.
This basic principle hasn't changed from the old color code, e.g. red sleeving on black to use as a phase, black sleeving on a blue to use as a neutral, etc.
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
That's an interesting one, since strictly speaking the I.E.E. Regs (since long before they became BS7671) have said that a single-phase circuit derived from 3-phase should always use the "basic" live color -- Formerly red, now brown. Use of the other phase colors (yellow & blue, now black & gray) is permitted on a single-phase branch feeder up to the final distribution panel though.