Dichromacy

A category of colour vision deficiency in which one of the three types of cone cells is completely absent, reducing colour perception to two primary channels instead of three. The three forms are protanopia (missing red cones), deuteranopia (missing green cones), and tritanopia (missing blue cones). Dichromacy is more severe than anomalous trichromacy, with the colour space effectively compressed to a single gradient line rather than retaining partial three-dimensional colour perception.