We had a couple of drivers who were colour blind, one managed to go full IFR later. He was quite a case and worth a story. Turns out he had a 'connection' to get a copy of the colour charts book. Came the crucial test; as the pages went by our subject confidently quoted the number on each chart.
The ol' doc looks at him fixedly, "you haven't got a clue have you?" he says. 'Yes', says our mate, 'that's number so and so on that page' as he points firmly at it.
The doc turns a few more pages, the penny drops, notes our mate is reading the page number, which most people are flat out seeing and then says, "y'know there are two numbers charts books that we use."