Colour blindness, also known as Daltonism, is a condition in which an individual has difficulty distinguishing between certain colours. It is caused by an inherited genetic defect that affects the colour-sensitive cells in the retina of the eye. People with colour blindness are unable to distinguish between certain shades of red, green, and blue, and may have difficulty seeing other colours. Colour blindness can range from mild to severe, and can affect both men and women.