Color vision deficiency means that your ability to distinguish some colors and shades is less than normal. It occurs when the color-sensitive cone cells in your eyes do not properly pick up or send the proper color signals to your brain.
About eight percent of men and one percent of women are color deficient. Red-green deficiency is by far the most common form and it results in the inability to distinguish certain shades of red and green. Those with a less common type have difficulty distinguishing blue and yellow. In very rare cases, color deficiency exists to an extent that no colors can be detected, only shades of black, white and grey.