Some people can't tell one color from another. They are said to be color-blind. The colors that cause most problems are red and green. To choose who have color blindness, both appear to be gray. Very few people have complete color blindness, in which everything appears of gray.
Studies show that boys are more likely to be color-blind than girls. One out of every seven boys has this problem. Studies also show that ten percent of the boys who live in large cities have color blindness. Only three percent of the boys who live in towns and in the country are color-blind. No one knows why color blindness is linked with living in big cities.