There are three types: two affect the red-green and the other, blue-yellow. Red-green colour deficiency is far from being the most common and reflects an inability to differentiate certain shades of red and green. Problems associated with seeing blue-yellow are very rare and reflects an inability to distinguish certain shades of blue and yellow. In very few cases, the inability to perceive colours is so severe that no colours are differentiated. In such cases, the person only sees their world in shades of black, white and grey.
