Cones pick up colour. Rods pick up brightness. There are blue cones, red cones and green cones. They pick up different wavelengths of light. Colour blind people have less numbers of particular cones than normal, so they get colours confused.
Some people are more colour blind than others.
Web accessibility issues for individuals who are color blind often involve color combinations that are not properly coordinated or do not provide high contrast. Images without alternative text are also an inconvenience, particularly when the individual is not able to discern what the image is due to the nature of his or her blindness. You can learn more about color blindness at the Lighthouse International Web site . Blindness comes in a variety of degrees. Most people defined as being blind often do have a measure of sight, as limited as it might be.