The iris is a muscular diaphragm around the pupil. It controls the amount of light entering the eye. Eye colour depends on the amount and type of pigments in the iris of the eye. Violet is the most rare eye colour. It is thought to be a lack of enough pigment in the iris to cover the blood vessels. This condition results in violet when light is reflected. After violet, green is the most rare eye colour.
Genetics affect eye colour and there are several genes involved in eye colour outcome. The genes of both the mother and father affect the probability of a child's eye colour. The main eye colours in decreasing commonality are brown, blue, grey and green. Beyond the main colours are variant shades of colour and combination colours. These include hazel, amber, violet and black (dark brown).
Changes in the eye colours are from genetic mutations. Some scientists think that blue eyes first started with one person living near the Black Sea. The ratio of worldwide eye colours has changed throughout time. This is because of different groups intermixing. Even in the last 100 years, the ratio has changed noticeably.
The populations of western Russia, the countries around the Baltic Sea and certain northern parts of the Scandinavian countries include a major portion of people with blue eyes. Those countries also include a significant amount of grey-eyed people. Blue eyes are found mainly in Europe and North America. In Asia, South America and the Middle Eastern countries, brown is the dominant eye colour. Green eyes are one of the rarest eye colours. Green-eyed people are mainly found in the U.K., Ireland and Continental Europe. However, in Iceland, green is the most common eye colour after blue.
Eyes tend to look different depending on the light quality and intensity. The colours of the room and a person's clothes can make eyes appear to be a different colour. Make-up also affects what the eyes look like. Eye colours can change with age and someone who has brown eyes can develop hazel eyes over time.
Geneticists have indentified certain genes with some eye colours. However, there are still major unknown factors in understanding eye colouration. Some of what is understood is that at least two genes are main factors in creating a person's eye colour. Also, the genes affecting eye colour also affect other aspects of a person's appearance. Completely different genes are associated with green grey and blue eyes with brown inclusions. Definitions for eye colours are sometimes used in more general terms, which can create confusion. The eye colour definition for hazel eyes usually means a combination of green, blue gold and brown, with irregular patterns. However, hazel is also used for other eye colours that cannot be defined elsewhere.