As a CVD pilot (in UK) I should like to add a few comments to what has been said about the many testing methods.
I have also had the "dear John" for Gatwick (although some time ago now) and know how it feels to have your lifelong dreams dashed in an instant. Following this I did get my PPL in the USA and did the light signal test - and passed
This gives me an unrestricted class 2 medical, good for a CPL in the USA.
Now for an ATPL you need a class 1 medical and will need to do a medical flight test at night with an FAA official - NOT a doctor. The FAA officials are all VERY enthusiastic about aviation and want you to pass, however they are also responsible for aviation saftey and will NOT pass anyone they feel is a danger to aviation, people do fail - it is not an automatic pass for CVD pilots.
In my mind and after reading a LOT about CVD I think it is a very complex matter and can be theorised for ever. The FAA route of a PRACTICAL test is appropriate, as if you can convince an FAA official that you are colour safe for AVIATION, I think this is a lot more realistic than a lot of theoretical tests. This also applies for other afflictions such as fasle limbs etc. If you can show the FAA guy that you can complete all the tasks that are needed for the safe operation of the aircraft, they will issue a SODA waiver (Statement Of Demonstrated Ability) - How practical!!
For myself, I intend to do the City Uni tests next month to see if I stand a chance here in the UK, if not will probably go back to the US and sit the medical flight test and go the FAA route. I'm not sure if this will be any good to me as my base is here in the UK, but feel that I must go as far as I can with my dream of flying.