Not really much point getting into the Ishihara debacle. Check the threads on medical if you want warts and all. But to sum it up...
The usage instructions for the Ishihara tests state that 2 or less errors indicate no colour blindness (very common for people with no deficiency to fail a couple). However the CAA (and associated EuroTrash regulators) seem to think they know better than the inventor and have decided it within their remit to disregard the instructions for performing an established medical testing procedure and will shoot you if you get a single plate wrong.