The development of eye color is established by the time a child is 2 years old, meaning that the iris has become fully pigmented. There are 4 factors that determine eye color:
- Sympathetic stimulation of the iris melanocytes (cells that produce melanin in the eye),
- Melanocyte stimulating hormone (MSH), and
- Biochemical factors that impact the metabolism of melanin.
An alteration in any of these 4 factors can cause the color of the iris to change. Alterations can occur during fetal development (congenital heterochromia) and/or due to injury or diseases/conditions (acquired heterochromia) that impact any of the 4 factors. In most cases, the color change occurs bilaterally (it impacts both eyes). However, in rare cases, the change in color impacts only one eye. This is called heterochromia.
When we look at the human eye, what we call eye color is the visible color of the structure we call iris. What gives color to iris is the color pigments on it. The darker the appearance of these pigments. In the eyes of blue people, these pigments are either very small or absent.