In color matching experiments, dichromats can match any color by a mixture of only two primary colors. A third, very rare form of color vision deficiency is called monochromatic . Monochromats cannot discriminate colors and perceive only shades of gray. There are two types of monochromats. Rod monochromats completely lack cones.
Cone monochromats have only one cone type.
They usually have good visual acuity and none of the other vision impairments found in rod monochromats. Besides these congenital color vision deficiencies, there are also acquired color vision deficiencies . These may be caused by illness or injury. Color discrimination also declines with age. Red-green defects occur in trichromatic and dichromatic forms.