Monochromats are able to match all spectral colors using just one color. These individuals are unable to discriminate solely on the basis of wavelength and are therefore truly color blind.
There are two types of monochromacy. Rod monochromats have normal rods but have no functioning cone receptors. As a result, their vision at night is normal but under photopic (daylight) conditions, visual acuity is poor and affected individuals tend to suffer from photophobia and nystagmus. Rod monochromacy affects about 0.001% of men and women.
Cone monochromats have one type of cone (normally blue). As a result, they often have poor acuity but in general, are less handicapped than rod monochromats. This condition is extremely rare.