Eye color is a polygenic trait and is determined by the amount and type of pigments in the eye's iris.^[Humans and animals have many phenotypic variations in eye color.^[In humans, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris.The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.

Three main elements within the iris contribute to its color: the melanin content of the iris pigment epithelium, the melanin content within the iris stroma, and the cellular density of the iris stroma.In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin.^[ Color variations among different irises are typically attributed to the melanin content within the iris stromaThe density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium.
 

Eye colors can range from the most common color, brown, to the least common, gray. Rare genetic mutations can even lead to unusual eye colors: black, red, and violet. Eye color is an inherited trait influenced by more than one gene.^[6][7] These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as single nucleotide polymorphisms or SNPs. The actual number of genes that contribute to eye color is unknown at present, but there are a few likely candidates.

The gene OCA2 (OMIM: 203200), when in a variant form the gene causes the pink eye color and hypopigmentation common in human albinism, (the name of the gene is derived from the disorder it causes, oculocutaneous albinism type II). Different SNPs within OCA2 are strongly associated with brown and green eyes as well as variations in freckling, mole counts, hair and skin tone. The polymorphisms may be in an OCA2 regulatory sequence, where they may influence the expression of the gene product, which in turn affects pigmentation.^[ A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes.Other genes implicated in eye color variation are: SLC24A4, TYR

Blue eyes with a brown spot, green eyes and gray eyes are caused by an entirely different part of the genome. As Eiberg said: "The SNP rs12913832 [of theHerc2 gene] is found to be associated with the brown and blue eye color, but this single DNA variation cannot explain all the brown eye color variation from dark brown over hazel to blue eyes with brown spots".

Eye color usually stabilizes when an infant is around 6 months old.
Synopsis

Schematic representation of different eye colors resulting from different conditions in the iris

Classification of colors

The perception of color depends upon various factors. These are the same eyes; however, depending on the light and surrounding hues, the eye color can appear quite different.

Iris color can provide a large amount of information about an individual and a classification of various colors may be useful in documenting pathological changes or determining how a person may respond to various ocular pharmaceuticals.