Acceptable line colours for single phase wiring?

In general you can have any colour that is not green and yellow, for any function, but if you are re-using non standard colours then at the ends, or at places where they may be accessed, such as junction boxes and fittings, then some brown oversleeve for lives and blue oversleeve for neutral is the correct practice.
Note that this situation is very common as part of 2 way light switches and things like over-run fan controls requiring both a steady live and a switched one, and in things like theatre lights where many circuits share a multi core cable. Also note that 'in practice' full adherence to the sleeving rules is far from universal.
Mike.