Why do people's eyes turn light blue?

The color of eyes is the most important physical feature of human body, but scientists say that the color of eyes can be changed. According to a recent report in The New York Times, under normal circumstances, the human eye may change color under the effect of trauma, but without the effect of trauma, will the eye color change automatically? For most people, the answer is no, the color of eyes has been formed in human childhood, and it is basically unchanged for life. However, the eye color of a few adults will obviously become darker or lighter with age. Scientists point out that it is melanin in the matrix in front of iris that determines the color of human eyes. The more melanin, the darker the human eye, and vice versa. However, the melanin content in the matrix usually remains unchanged for life, unless something abnormal happens to cause it to change permanently. First of all, eye diseases, such as glaucoma, change the melanin in the eyes. Secondly, the color eye caused by trauma (also known as binocular heterochromia), which is mainly manifested in the difference of eye color. For example, david bowie, a famous British rock singer, has a light blue eye and a brown eye. It is reported that Bowie's eyes changed color because of facial injury in childhood. Finally, heredity is also the cause of eye color change. After studying thousands of pairs of twins in 1997, scientists found that 10% to 15% of twins' eyes gradually changed in adolescence and adulthood, which proved that twins with similar genes had the same probability of color change. Scientists finally clearly pointed out that the key to the color change of human eyes is heredity and trauma.