This illustration from 1895 shows the US flag as it seen by those with normal vision, and also those with different types of color blindness

People with color blindness cannot tell the difference between certain colors. They may not see colors at all.

Most color blindness is heritable, usually as simple Mendelian inheritance. Sometimes, it is the result of damage to the eyes, nerves, or the brain. It can be caused by coming into contact with certain chemicals.