As the perception of color is dependent on viewing conditions (e. g. the amount and type of illumination, as well as the hue of the surrounding environment), so is the perception of eye color.^[14]

Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue.^[6] Seeing the need for a standardized classification system that was simple, yet detailed enough for research purposes, Seddon et. al developed a graded one based on the predominant iris color and the amount of brown or yellow pigment present. There are 3 true colors in the eyes that determine the outward appearance; brown, yellow, and blue. How much of each color one has determines the appearance of the eye color. The color of the eyes in turn depends on how much of these colors are present. For example, green eyes have yellow and some blue, making them appear green. Brown eyes appear brown because most of the eye contains the brown color. The above is true for Homo sapiens; the iris color can vary in the animal world. Instead of blue in humans, autosomal recessive color in the species Corucia zebrata is black, whereas the autosomal dominant color is yellow-green.^[15]

 Amber

Human amber eyes displaying the yellow pigments.

Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This might be due to the deposition of the yellow pigment called "lipochrome" in the iris (which is also found in green and violet eyes).^[16][17] They are nicknamed "Wolf eyes" due to the high rate of the amber eye color in wolves. Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise of many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples; while amber eyes are of a solid gold hue.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines.^[18] The bright yellow eyes of the Great Horned Owl are thought to be due to the presence of the pteridine pigment xanthopterin within certain chromatophores (called xanthophores) located in the iris stroma.^[19] In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.^[20]
 

Blue

A blue eye

Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of light tend to be absorbed by the underlying iris pigment epithelium, and shorter wavelengths are reflected and undergo Rayleigh scattering.^[5] The type of melanin present is eumelanin.^[21] The inheritance pattern followed by blue eyes is considered similar to that of a recessive trait, however it is a polygenic trait (meaning that it is controlled by the interactions of several genes, not just one).