Monochromacy or (achromatopsia)
This form of complete color blindness is a very rare subtype of color vision deficiency among much more common ones like the the well known red-green color blindness. People suffering under monochromacy, perceive everything just in shades of gray.
Monochromacy, more commonly referred to as "total color blindness", is caused by the total absence of either 2 or 3 of the pigmented retinal cones, reducing vision to one dimension. Monochromacy comes in two forms:
Rod monochromacy: is a rare, non progressive inability to distinguish any color, resulting from non-functioning or absent retinal cones. Rod monochromacy is typically associated with sensitivity to light (Photophobia) and poor vision.
Cone monochromacy: is also a rare type of total color blindness, however is accompanied by relatively normal vision.