Colour vision deficiency (sometimes called colour blindness) represents a group of conditions that affect the perception of colour. Red-green colour vision defects are the most common form of colour vision deficiency. Affected individuals have trouble distinguishing between some shades of red, yellow, and green. Blue-yellow colour vision defects, which are rarer, cause problems with differentiating shades of blue and green and cause difficulty distinguishing dark blue from black. These two forms of colour vision deficiency disrupt colour perception but do not affect the sharpness of vision.