The Regs. here basically allow any conductor to be sleeved at its termination points, except that a green/yellow wire may not be re-identified and used for any purpose other than earth.
So you can take a blue, black, gray, etc. wire, sleeve it green/yellow at its ends and use for an earth, but you cannot take a green/yellow wire and sleeve it blue, brown etc. for use as a phase or neutral.
The reasoning is that if somebody comes along later and cuts into an existing cable, he might assume that the Gn/Yw is earth if he doesn't check the ends and see that it's been reidentified.
Of course, with our "T&E" PVC-insulated cables commonly used in residential and light commercial wiring the earth wire is bare inside the sheath, and may not be used for any other purpose except an earth anyway.
