A mate of mine is colourblind, I'm not sure of his exact condition and the extent of his impairment but he works as a day fitter/technician of some description (I'm not sure his exact role!) for a freight operator.
I do know his impairment is enough to prohibit him from safety critical rolls such as driving, dispatching and guard.
I'm aware he applied at a TOC for a similar role previously and was knocked back due to his impairment with them, but I think it depends on the roll, the safety critical related duties involved and of course presumably roles which involve colours such as wiring would probably be a no too.
He has previously worked as a revenue inspector too.
I think the best bet is to make them aware of the condition and the extent to which it affects you when appropriate, and definitely make sure they know at the medical stage!