There are some test plates for which numbers only show up for people with a particular colour deficiency. Or so I was told once by a tech, when I admitted that I couldn't see anything at all on a particular plate.
Some of the plates have two "answers": the one that is a different colour than the background, and another number that is distinguished from the background by differences in greyscale. People with normal colour vision see the coloured number effortlessly and go to the next page. People with a colour deficiency who hunt for the number using other clues, trying to pass the test, will fall into the trap and spot the grey number.