BS7671 and BS3858 - Blue sleeving on a live conductor in a multipolar cable, in order to use it as neutral

In a multipolar cable (black, brown, grey and CPC), i need to apply a blue sleeving on the grey conductor, at its terminations, and use it as neutral

Is doing so compliant to the regulations (BS3858 and BS7671)? And if so, which sentence or point of the regulations specifies it?