monochromasy

from The Century Dictionary.

• noun In psychology, that form of color-blindness in which all visible objects are seen as shades of the same quality. As opposed to dichromasy, by the adherents of the Young-Helmholtz theory of color-vision, the term leaves it open as to whether the single remaining visual quality is a color or a brightness. There is now, however, no doubt that monochromasy is identical with total colorblindness, and that the vision of the monochromate is gray-vision and that only.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun complete color blindness; colors can be differentiated only on the basis of brightness