If it is of help to you here is my take on it.
I have a colour vision defect.
I researched and eventually went to Gatwick specifically for the UK CAA Class 1 Medical on the agreement i sit the colour assesment first.
If I failed the colour assesment I would pay the £40 and be on my merry way, else they would continue and complete the full medical.
That was 6 years ago, and I am now a skipper.
My advice to you as with all things medical, is you can ask people here till you are blue int eh face, but the Aeromedical centre is the only place you will get a diffinate answer.
Its harsh if you fail kinda thats it, but you will never know till you go up there and try.
True
[email protected] fo your question. or 01293573700
or If your going for Irish contact Dr Annetje Roodenburg in the IAA Aeromedical Section. She has a reputation but can be very helpfull.
Best of luck