Brown eyes have more pigment than blue eyes, which have little or almost no pigment in them, the lighter the shade. "Brown eyes are predominant in humans[37] and, in many parts of the world it is nearly the only iris color present.[38] It is less common in countries around the Baltic Sea and in Scandinavia. Dark pigment of brown eyes are most common and with a few exceptions the only color among the population of East Asia. In humans brown eyes contain large amounts of melanin within the iris stroma, which serves to absorb light at both shorter and longer wavelengths. Brown eyes are the most common eye color, with over half of the world's population having them." (http://en.wikipedia.org/wiki/Eye_color#Brown)
Hazel eyes remain complex. "Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer.[7][25] Hazel eyes often appear to shift in color from a light brown to a dark golden-green. A number of studies using three-point scales have assigned hazel to be the medium-color between the lightest shade of blue and darkest shade of brown. Hazel mostly consists of Brown and Green. The dominant color in the eye can either be green or light brown/gold. This can sometimes produce a multicolored iris, i.e., an eye that is light brown near the pupil and charcoal or amber/dark green on the outer part of the iris (and vice versa) when observed in sunlight. Hazel is commonly found in Europe, the Middle East, Americas, Central Asia and parts of South Asia.
Definitions of the eye color hazel vary: it is sometimes considered to be synonymous with light-brown or gold.[46][48][51][53] In North America, hazel is often used to describe eyes that appear to change color." (http://en.wikipedia.org/wiki/Eye_color#Hazel)
"The question of Hazel eye colour has haunted the literature," muses geneticist Rick Sturm, principal research fellow at the University of Queensland in Australia. "The fact is eye color (like skin and hair color) is a continuous spectrum — from the lightest shades of blue to the darkest brown/black." See figure for a hint at the variation.
Moreover, it's subjective. "Visual impressions of colour can change as often as you change the lighting conditions."
Also, hazel eyes, like blue eyes and all light eyes, reflect colors around them, as a pool reflects the sky.
Sturm and his team have studied adolescent twins to determine the genetic underpinnings of eye color. In the course of these studies, he's taken 1,937 eye photographs, and divided the group into three major eye-color definitions. Sixty-percent of the twins fall in the 'blue' category, 26% in 'green/hazel' and 14% 'brown.'
"Surprisingly," 74% of the green/hazel eyes had a brown ring around the pupil. "This major pattern may explain a lot of eye color that is commonly referred to as Hazel."
These statistical findings are unpublished, and not yet accepted by peer review; they "only represent my opinion at this stage" says Sturm.
How do such eyes occur? Certainly the simple model we learned in school about brown-eye color being dominant over blue falls short of an explanation. Indeed that one-gene theory is kaput. There is no single gene for eye color. Now, we know two major genes and other minor ones account for the tremendous variation of human eye color, says Sturm, part of the team making this discovery, reported in 2007.
The gene OCA2 produces a protein that allows the hair, skin and eyes to make pigment (called melanin) that colors these body parts. The more pigment in the eye, the darker it is. Much pigment results in brown eyes; little pigment causes blue eyes.
Furthermore, a change happens fairly frequently to the pigment protein under the control of the OCA2 gene. When the protein changes, its function changes. It makes a different pigment that then colors the eyes green or hazel. Sturm likens this process to "changing a light bulb from brown to green." " (http://images.search.yahoo.com/images/view?back=http%3A%2F%2Fsearch.yahoo.com%2Fsearch%3Fei%3DUTF-8%26p%3Dhazel%2Beyes%26b%3D1%26xargs%3D0%26pstart%3D1%26xa%3D5_FyDxn7I6bokQ_0NkRaGQ--%252C1253570616&w=192&h=191&imgurl=www.wonderquest.com%2F2007-03-19-sturm-eyes.jpg&size=17.4kB&name=2007+03+19+sturm+eyes+jpg&rcurl=http%3A%2F%2Fwww.wonderquest.com%2Fmemory2.htm&rurl=http%3A%2F%2Fwww.wonderquest.com%2Fmemory2.htm&p=hazel+eyes&type=jpeg&no=1&tt=11%2C649&oid=46c054c5648348ee&tit=2007+03+19+sturm+eyes+jpg&sigr=116cnl0hu&sigi=11dosdoiv&sigb=13itnlb0p)
One of the many varieties of Hazel eyes...
This is the broad spectrum of eyecolor, showing the most common:
