People who cannot tell all colors apart are said to be color-blind. Most color-blind people can see yellows and blues, but confuse reds with green. It is very rare for a person to be blind to all colors, but they may see everything in shades of black, white and gray.
It is interesting to point out that many color-blind people don't even realize that they are color-blind, they don't know that the colors they are seeing and naming are not the actual colors that people with normal vision can see. This can be dangerous when a color-blind person confuses the red and green of a traffic light.
Color blindness is thought to be inherited and although doctors have tested color blindness, there is no cure to treatment for it.
It's especially dangerous for a color-blind person to cross a street when  _
it is crowded it is a dark night there are no traffic lights at the cross of the streets the traffic light turns red