Color blindness
Many people do not understand color blindness. People who are color blind never swap the colors they are blind to.  They may have trouble telling two colors from one another. That way they may have a problem finding the right kind of apple in the supermarket. The image below first shows the way that two apples look to a person with normal vision, and then the way it looks to a person with red-green color blindness. The left apple is a Braeburn; it is red in color. The apple on the right is a Granny Smith; it is green. To someone with red-green blindness, the apples look like they are almost the same color.