<P> The optical mechanisms by which the non-pigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature . Simple selective absorption and reflection by biological molecules (hemoglobin in the blood vessels, collagen in the vessel and stroma) is the most important element . Rayleigh scattering and Tyndall scattering, (which also happen in the sky) and diffraction also occur . Raman scattering, and constructive interference, as in the feathers of birds, do not contribute to the color of the human eye, but interference phenomena are important in the brilliantly colored iris pigment cells (iridophores) in many animals . Interference effects can occur at both molecular and light microscopic scales, and are often associated (in melanin - bearing cells) with quasi-crystalline formations which enhance the optical effects . Interference is recognised by characteristic dependence of color on the angle of view, as seen in eyespots of some butterfly wings, although the chemical components remain the same . White babies are usually born blue - eyed since there is no pigment in the stroma, and their eyes appear blue due to scattering and selective absorption from the posterior epithelium . If melanin is deposited substantially, there will be brown or black color, if not, they will remain blue or gray . </P> <P> All the contributing factors towards eye color and its variation are not fully understood . Autosomal recessive / dominant traits in iris color are inherent in other species but coloration can follow a different pattern . </P> <P> Amber colored eyes are extremely rare in humans . They consist of a solid orange / gold color that may contain lighter shades of the same pigment within the iris . This is an unusual occurrence that happens when the yellow pigment called pheomelanin (also known as lipochrome), is dominant within the iris . Pheomelanin is also found on individuals with green eyes in much smaller amounts . This is because green eyes have a strong presence of both melanin and pheomelanin . Often in poor lighting, one may mistake amber eyes for brown . This also happens when viewed from far away or in pictures with poor lighting as well . However, in natural or well lit areas, it is very easy to tell the difference between the two colors . Another common mistake people make is referring to amber eyes as hazel . Although similar, hazel eyes have a stronger presence of melanin with two very distinct colors within the iris (usually green / brown) and often contain many speckles or blotches of mixed hues . </P> <P> Heterochromia (also known as a heterochromia iridis or heterochromia iridum) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). Uncommon in humans, it is often an indicator of ocular disease, such as chronic iritis or diffuse iris melanoma, but may also occur as a normal variant . Sectors or patches of strikingly different colors in the same iris are less common . Alexander the Great and Anastasios the First were dubbed dikoro * s (dikoros, "with two pupils") for their patent heterochromias . In their case, this was not a true dicoria (two pupils in the same iris). Real polycoria can be due to disease but is most often due to previous trauma or surgery . </P>

In the visual system the opening in the center of the iris is known as the