The CAA policy on defective colour vision for Class 2 medicals appears to be as follows:
"You will be tested for normal colour vision with Ishihara Test Plates. If you fail these you can still gain a Class 2 certificate, but it will be limited to flying by day only (previously you would also have been restricted to VFR flying in JAA airspace, but this has now been dropped, and the limitation is standard ICAO). This limitation can be removed if you pass an approved colour lantern test. If you wish to take a lantern test you should contact the CAA Medical Division, Gatwick, to get details of the nearest lantern to you."