Historically the UK used its own set of colour codes for fixed and flexible wiring. In the early 1970s the colours used for flexible wiring were harmonised across Europe, however the colours for fixed wiring remained unchanged until recently. In March 2004 the fixed wiring colours were also harmonised. During a transition period that began on 31st of March 2004 and ended on the 31st March 2006, use of either colour scheme was acceptable. Now only the harmonised colours may be used. The result is that it is now possible to encounter installations that use both colour schemes, and hence great care must be taken to ensure conductors are correctly identified when carrying out any work.

 
Notes

1. Earth wire in cable will be a bare conductor and will need green/yellow sleeving to be applied at point of termination.
2. Earth wires are present in T&E or 3&E cables. Other cable types such as SWA or MICC will still need to be earthed, however this will be carried using the cable armour / screen.

There is an IEE leaflet explaining these changes. That is available here

Installations that are wired using cables to both colour schemes should carry a warning sticker (see example) on or near the consumer unit that statesand looks something like this: