If any of the three types of cones are missing or non-functional, the full range of color cannot be perceived. An inability to perceive the normal range of colors is called a color deficiency. A sex-linked trait, color deficiency occurs more often in males than females. There are many levels of color deficiency and a variety of causes.
Monochromats are totally color-blind.
In this category, individuals may have all three types of cones missing or defective, or they may have only one type of functioning cone. They are unable to distinguish one color from another. This would be analogous to watching a color television set, but only seeing the image in black and white. This level of color deficiency is extremely rare. In red-green color deficiency, the individual cannot readily distinguish between red and green light.