Color blindness (color vision deficiency) is a condition in which certain colors cannot be distinguished, and is most commonly due to an inherited condition. Red/Green color blindness is by far the most common form, about 99%, and causes problems in distinguishing reds and greens. Another color deficiency Blue/Yellow also exists, but is rare and there is no commonly available test for it.
Ishihara Test for Color Blindness
What numbers do you see revealed in the patterns of dots below?