mrg92
If you fail the ishihara test, ie one or more errors then you will go straight to the CAD test. It will make it quite a day to say the least!
If you feel that you have CV problems - GET SOME PRACTISE. Buy your own ishihara plates and study them just as Nemesis666 says.
You can also just ask Gatwick for a colour vision test - I think it is about £120 now and IF you pass then book your C1 and will not have to do CV tests again.
Note the IF above. The CAD test goes straight for our weaknesses and is quite a horrible experience in the process. I can tell you from my own experience that after failing the ishihara, you know your whole future rests on a 15 minuet test (less when it was the lanterns) and that puts you under immense stress. If you fail the CAD - your aviation career is OVER there and then, it is that simple. You will walk out into the car park and see the jets above you and feel as though you have just been killed.
Do what is right for you and get PRACTISING now - GET YOUR C1.
Oh and as a matter of interest to all, the CAD test is planned to expand to be used as a screening test. Basically a shortened version INSTEAD of the ishihara to stop people from doing their own research before hand so to speak. So get in quick!!!