Colour blindness (colour vision deficiency) is a condition in which certain colours cannot be detected. There are two types of colour vision difficulties: congenital (inherited) problems that you have at birth, and problems that develop later in life.

People born with colour vision problems are unaware that what they see is different from what others see unless it is pointed out to them. People with acquired colour vision problems are aware that something has gone wrong with their colour perception.

Congenital colour vision defects usually pass from mother to son. These defects are due to partial or complete lack of the light-sensitive photoreceptors (cones) in the retina, the layer of light-sensitive nerve cells lining the back of the eye. Cones distinguish the colours red, green, and blue through visual pigment present in the normal human eye. Problems with colour vision occur when the amount of pigment per cone is reduced, or one or more of the three cone systems are absent. This limits the ability to distinguish between greens and reds, and occasionally blues. It involves both eyes equally and remains stable throughout life.

There are different degrees of colour blindness. Some people with mild colour deficiencies can see colours normally in good light but have difficulty in dim light. Others cannot distinguish certain colours in any light. In the most severe form of colour blindness, everything is seen in shades of gray.

Except in the most severe form, colour blindness does not affect the sharpness of vision at all. It does not correlate with low intelligence or learning disabilities.

Most colour vision problems that occur later in life are a result of disease, trauma, toxic effects from drugs, metabolic disease, or vascular disease. colour vision defects from disease are less understood than congenital colour vision problems. There is often uneven involvement of the eyes and the colour vision defect will usually be progressive. Acquired colour vision loss can be the result of damage to the retina or optic nerve.

There is no treatment for colour blindness. It usually does not cause any significant disability. However, it can prevent employment in an increasing number of occupations.

Change in colour vision can signify a more serious condition. Anyone who experiences a significant change in colour perception should see an optometrist or ophthalmologist.