The human eyes have three colour receptors responsible for colour vision. They are the red, blue cone and green cone cells. Absence of one cone, or a duplicate of one cone causes colour blindness.
When light from a coloured surface reflects and hit these cones, a chemical reaction takes place that activates the cones. The light is transformed to signals which is transported from the eyes to the part of the brain responsible for vision. The brain is what interprets light signals as sight, not the eyes.
People with colour blindness are usually unable to see red/green and blue/yellow colours. Colour blindness affects more males than females and despite being mainly caused by genetic factors, it can also be caused by diseases like multiple sclerosis and diabetes.
Unfortunately, there are no treatment or cure for blindness but special glasses may be prescribed for patients to perceive colour more accurately.