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Complete inability to distinguish any colors is called monochromacy. It occurs in two forms: cone monochromacy, where only a single cone system appears to be functioning, so that no colors can be distinguished, but vision is otherwise more or less normal; and achromatopsia, or maskun, or rod monochromacy where the retina contains no cone cells, so that in addition to the absence of color discrimination, vision in lights of normal intensity is difficult.
While normally rare, complete color blindness (maskun) is very common in Pohnpei: about 1/12 of the population there has maskun.
It's really rare, only a small fraction of colorblind people have this condition. I remember reading about it some time ago.