It is true that legally blindness is defined differently. We often refer to people who misperceive colors as being color blind. That may or may not be correct. For example, if a person has no cone receptors they are truly color blind. Such people are called achromats and are very rare.
Some people have rods and one kind of cone.
These people are also unable to make any discriminations based on color and are called monochromats. In the section dealing with the trichromatic theory I explained that most people have three classes of cone receptors in their retina . These are the short wavelength, middle wavelength and long wavelength receptors. Such observers are called trichromats. Some people are born with only two classes of cone receptors.