They use the Ishihara test. Bad flourescent lighting and untrained nurses can lead to a false indication of color deficiency. You can't stare at the plates too long or you WILL detect a pattern. Had the same thing on my first go and had to re-test. Fortunately with a trained nurse outside in natural light. Even she said she could see the pattern on the one you're not supposed to see if she stared at it long enough.
oh, and you can't become color blind. You either are color blind or you are not. It doesn't change.
TP