One day at school I was day dreaming out of the window. All of a sudden I heard my name.

Teacher: "Brad, what colour is that on the right page?"

Me (Turning to the page): "Red."

Teacher (Mad): "No! What colour is it really?"

Me: "Green?" (Now I knew if something wasn't red it must be green)

Teacher (Furious): "Stop playing games! What colour is it?"

Without red and green, I had no clue. I started spouting out colours.

Me: "Orange? Yellow? I don't know. I honestly don't know."

Teacher: "It is brown! It is brown!"

"Brown," I thought," I would never have guessed brown!"

Many everyday tasks depend on the perception of colour, and people with colour blindness, or, more accurately, colour vision deficiency are challenged by tasks as simple as reading colour-coded information such as maps or charts, selecting ripe fruit and vegetables, identifying medication that is poorly labelled, or coordinating colours when buying clothes or deciding what to wear each morning. As in the example above, it can be particularly difficult for children at school where educational material is often colour-coded, or if they are unable to read a green board when yellow chalk is used.

People who are able to perceive colours are often puzzled by the way in which colour blindness affects activities such as driving and the interpretation of traffic lights, for example, but it is possible to compensate and respond to other cues, such as the position of the lights. Most people with colour vision deficiency adapt fairly easily to their different perception of colour, because what we see is not only dictated by the actual light coming into our eyes, but by the knowledge in our heads and our life experiences. What is colour blindness?

Although the term "colour blindness" is commonly used, it is extremely rare for people not to see colour at all, and the term "colour vision deficiency" is more accurate. People with colour vision deficiency find it difficult to identify and distinguish between certain colours. They are not aware of differences between colours that are obvious to the rest of us. Because colour perception is subjective, many people may be unaware of their condition unless they are tested for it. It is seldom an indication of something more serious, but is simply a different view of the world.

How do we see colour?

Colour vision depends on the eyes and the brain working together to perceive different properties of light. Vision begins when light enters the eye, and the cornea and lens focus it onto the retina at the back of the eye, which contains millions of light-sensitive cells called photoreceptors. Some photoreceptors are shaped like rods and some are shaped like cones, both of which contain photopigment molecules which respond to light in different ways. In each eye there are approximately 120 million rods compared to only 6 million cones. The signals picked up by the photopigments are sent via the optic nerve to the visual centre in the brain. The eyes see, but the brain processes. Because the human eye and brain together translate light into colour, each of us perceives colour slightly differently,

Rods contain only one photopigment, while cones contain one of three different photopigments. This makes cones sensitive to long (red), medium (green), or short (blue) wavelengths of light. The presence of three types of photopigments, each sensitive to a different part of the visual spectrum, is what gives us our rich colour vision.

Our colour vision is the result of hundreds of thousands of years of evolution under the light of the sun. Artificial lighting has a different colour temperature or cast to it, which is why even to a person with normal colour vision, colours can look different under indoor lights than they do outdoors in the sun. Photographers, film producers, artists and other people involved in any industry involved with graphic arts are well aware of this situation, and use it to their advantage in everything from advertising to product presentation on store shelves and more.

Is colour vision deficiency the same for everybody?

There are three main kinds of colour vision deficiency. The most common is red-green colour vision deficiency, and these people have difficulty distinguishing between reds, oranges, yellows, browns and greens. They may see these colours as duller than they would appear to people with normal colour vision, and find it a challenge differentiating shades of purple. It affects about 1 in 12 men and 1 in 200 women.

Far less common is blue-yellow colour vision deficiency, difficulty with blues, greens and yellows. Blue appears greener, and yellow may appear violet or light grey. It affects males and females equally.

A complete absence of colour vision is extremely rare, and may be accompanied by problems with visual acuity.

What causes colour vision deficiency?

In the vast majority of cases, colour vision deficiency is caused by a genetic fault passed on to a child by the parents. Mothers are usually the carriers of the genetic fault, passing it on to their sons, even though they don't have colour vision deficiency themselves. Fathers with colour vision deficiency will only pass it on to their children if their female partner is a carrier of the genetic fault.

Occasionally, colour vision deficiency may develop later in life as a result of an underlying health condition, a side effect of medication, or exposure to harmful chemicals. Some people find it difficult to distinguish between colours as they get older.

How is colour vision deficiency diagnosed?

Optometrists use a variety of tests to diagnose the different types of colour vision deficiency. Some of the tests require the identification of a shape within a pattern of different coloured and sized dots, while the goal of others is to arrange different blocks or pegs of similar colour in order of hue.

Can colour vision deficiency be treated?

There is no cure for colour vision deficiency, and most people adapt to it in their daily lives. However, special lenses and iPhone apps are available to help people perceive and discriminate between colours more accurately. These kinds of apps can be helpful in selecting ripe fruits such as bananas, or finding complementary colours when picking out clothing.

If a child has difficulty with colour vision, it may be helpful to inform the school, so that learning materials can be modified accordingly.

Famous people with colour vision deficiency

Have you ever wondered why the Facebook logo is bright blue? This is the colour chosen by Facebook founder, Mark Zuckerberg, to compensate for his red-green colour vision deficiency. Paul Newman with his piercing blue eyes had colour vision deficiency, as does actor Eddie Redmayne, golfer Jack Nicklaus, writer Mark Twain, President Bill Clinton, and movie director Christopher Nolan, among many others.

To quote Albert Schweitzer, "An optimist is a person who sees a green light everywhere, while a pessimist sees only the red stop light. The truly wise person is colour blind."