I think at the moment a Class 1 medical requires full colour vision. I also can't see the Ishihara plates, but for my class 2 was referred to the doc to do a Holmes lantern test which I passed....that at the time (2002) meant I could still get a class 1.
Basically there's different types of colourblindness, and from memory the major one which rules out a class one, is the inability to distinguish red from green. The other type, which I have, is the inability to distinguish as different shades of green and different shades of red i.e. you fail the plates because your eye isn't as good as distinguishing different levels of the same colour, however there is no confusion between the two colours. As such, I suggest you go down this route or seek advice from a specialist as to which further tests you can do as the Ishihara one will be failed by both types.
CTG