If it is any help: in the late 60'ies my father failed the Ishihara test as expected. but with the help of a friendly BelgCAA examiner, we were allowed to borrow a signal light for 'training'. So, over a couple of weeks he 'learned' to identify the colours of the light correctly. We sent him to the other side of the airfield and sent a series of signals that he wrote on a piece of paper. Repeated until he got it right. Took several weeks.
He continued flying until he lost his medical at 85.
I think , because he knew he was colourblind, that he never even attempted to identify colours before we 'trained' him. Worth a try.