I've always been of the opinion that you can sleeve any colour insulated conductor and use it as what ever you fancy. IE an insulated earth could be sleeved blue and used as neutral, and you could sleeve a brown to use it as earth, the point being that correct sleeving always this. Can't think of anything that says you can't use an insulated earth as a live core. You can't use a cpc in FTE as this si un-insulated as pointed out above