The Martin–Schultz scale is a standard color scale commonly used in physical anthropology to establish more or less precisely the eye color of an individual; it was created by the anthropologists Rudolf Martin and Bruno K Schultz in the first half of the 20th century. The scale consists of 20 colors (from light blue to dark brown-black) that correspond to the different eye colors observed in nature due to the amount of melanin in the iris:
References
Martin–Schultz scale Wikipedia(Text) CC BY-SA