Have you ever wondered why everyone's eye color is different from yours? Have you wondered why your eyes are brown, while the next person's eyes are blue? There is a reason.
Peoples of the earth for a very long time lived near the center of the earth where the equator is closest to the sun. Being closer to the sun means that there is more direct sunlight on your skin and face, coming into your eyes. If you look around in nature, you will notice animals, namely predators (i.e. cheetahs) who have dark rings of fur around their dark eyes. This is to help absorb both short and long wavelengths so that they are not blinded by the light bouncing off of their lighter fur. That is what helps them see their prey so well. Dark eyes are meant to absorb sunlight so that it is easier to see past all of the rays shining down. If peoples in the belt of the earth had brighter colored eyes, they would have a harder time seeing because their eyes would be refracting that light back off. The closer to the middle of the earth you get, you will notice, the darker the brown gets to be. Thousands of years ago, when peoples were still hunters and gatherers for survival, it was necessary for them to see in large amounts of sunlight - they did not have sunglasses; their eyes served as their sunglasses. This given-ability occurs because of a high concentration of melanin to the front of the iris, known as the stroma. Dark brown eye color is actually the dominant eye color in the human race, and more people in the world have brown eyes than they do all other colors combined; all other colors are recessive by nature.
So what about blue eyes? It is estimated, through genetic coding and understanding of iris pigmentation, that roughly 10,000 or so years ago, the first person existed on earth with blue eyes. He seems to come from an area above the Baltic Sea (Sweden, Finland, Norway, etc.). In that region of the earth, sunlight exists, but moreso does night take reign. It is not uncommon for people in that region to experience 8 months of 20 hour nights, and 4 hours of daylight. Daylight is not uncommon, but is very scarce in the winter months; this becomes the furthest point from the sun. There, thousands of years ago, it was a hindrance for hunters and gatherers to have dark-colored eyes. They did not need to absorb light so that they could see past it, they needed to reflect light so that they could see through darkness - the reverse of brown-eyed people. Over time, the genetic mutation occurred, and brown eyed peoples became blue-eyed peoples. This mutation is possible as the body learned to deposit small amounts of melanin into the stroma. Longer wavelengths are absorbed into the back of the iris (called the epithelium) and shorter wavelengths are scattered by the stroma for clearer vision. The color of blue eyes is not a pigment found in the iris; this is the same scattering (Rayleigh scattering) that makes the sky appear to be blue. If you think about it, it makes sense that people who live in the middle parts of the earth, or whose ancestors are from the middle parts of the earth, have darker or brown eyes. This also makes sense in explaining why people with lighter, or blue-colored eyes are mostly descended from the northern-most parts of Europe. As for gray eyes, they are another pigment of blue; they refract light, but absorb color waves. People with gray eyes can sometimes be told that their eyes look more green on a particular day, then blue on another day, and gray on the next. Blue eyes have a thin stroma and a dark epithelium. The difference between blue eyes and gray eyes has to do with a larger deposit of a pigment called collagen to the stroma in the gray eye. Because of the collagen deposits, the light hitting the stroma in the gray eye undergoes something called Mie scattering, in which longer wavelengths are scattered more than short wavelengths, as in blue eyes.
Green eyes are a bit more difficult to understand: green eye color comes from a pigmentation of light brown or hazel in the iris, paired with a dark epithelium, and blue light through Rayleigh scattering in the pigment. I like to think of green eyes as a good mix of every eye color; it has iris pigment, reflects short and long wavelengths, and absorbs short and long wavelengths. Out comes a beautiful green that many people have. Because of the prevalence of green eyes in Northern and Central Europe, mostly of recent Celtic descent, I believe it is possible that green eyes are another mutation of blue regaining brown pigmentation to handle the longer days of light. I could be wrong, but it makes a lot of sense!
Obviously, there are sundry eye colors that are not mentioned here, but take this knowledge and go find out more! Think differently when you see people; if they have blue eyes, they have an ancestor from the Northernmost parts of Europe; if they have brown eyes, they have ancestors from sunnier climate, near the middle of the earth. It's fascinating to know that we were built with bodies that could mutate our genetic code to meet our needs for survival: giving us our own personal sunglasses, or mutating to be a mobile, personal, quasi-flashlight. What color eyes do you have?