Why do humans have different eye colours?

Our eye colour is one of the most noticeable genetic features. Whether a person’s eyes are black, brown, blue, grey, green, hazel, or a mixture of these hues, they factor heavily in deciding the aesthetics of their features. Brown eyes are found in 60-90 per cent of the world's population of more than 7.5 billion individuals. In fact, 10,000 years ago, every human was assumed to have brown eyes. 

What gives eyes their colour? 
A person’s eye colour results from pigmentation of a structure called the iris, which surrounds the small black hole in the centre of the eye (the pupil). It helps control how much light can enter the eye. Eye colour is determined by variations in a person’s genes. Most of the genes associated with eye colour are involved in the production, transport, or storage of a pigment called melanin. Eye colour is directly related to the amount and quality of melanin in the front layers of the iris. People with brown eyes have a large amount of melanin in the iris, while people with blue eyes have much less of this pigment. Fun fact, melanin is also responsible for determining the colour of our skin! 

Can the eye colour of a baby be predicted? 
Eye colour was assumed to be passed through a single gene from each parent even when your parents or grandparents were in school. Science teachers used to teach pupils how to use a simple chart to predict eye colour. Scientists now know that the inheritance of eye colour is significantly more complicated than previously thought, involving more than one gene. As a result, predicting the colour of a baby's eyes is difficult. Eye colour isn't like paint, a baby's eye colour isn't simply a blend of its parents. Scientists do know, however, that if both parents have brown eyes, the infant is likely to get them as well. In addition, if both parents have blue eyes, the infant is likely to have blue eyes as well. 

Is it possible to have two different colours of eyes?
Occasionally we see a dog, horse, cow, sheep, rabbit, or cat with one brown eye and one blue eye. But this trait isn’t just unique to these animals, in rare cases, humans have it too. This condition is known as heterochromia iridis. In people with complete heterochromia, the iris of one eye is a different colour than the iris of the other eye. Segmental heterochromia occurs when areas of the same iris are different in colour. It may be a genetic trait or can develop further along in life due to some eye disease. 

Does the colour of your eyes change over time? 
The eyes can naturally change their colour as a response to the iris expanding or contracting in the presence of light or as the iris ages. This results in the eyes gradually becoming darker or lighter in colour. However, while eyes changing colour is a natural phenomenon, changes in eye colour that occur suddenly could indicate a medical problem that should be explored by a doctor.