| Not unlike snowflakes and fingerprints, human eyes are never exactly alike from person to person — in terms of color, that is. While brown may be the most common eye color, there are so many shades of it — not to mention blue, green, and other hues — that no two irises are identical, even among identical twins. Eyes get their color from the two-layered iris, with the back layer (officially known as the pigment epithelium) almost always containing brown pigment. The amount of pigment in the front layer (stroma) usually determines a person's eye color — a lot of brown pigment results in brown eyes, whereas people with blue eyes have no pigment at all in their stroma. Those with just a bit of pigment end up with green or hazel eyes. And there's always just the tiniest amount of variation in the results: If you've ever tinkered with a color slider, you'll have a better understanding of how rare it is for any two colors to actually be the same, even if the difference between them is so minute as to be barely detectable by, well, the naked eye. Then there's heterochromia — when one person has eyes of two different colors. Complete heterochromia is when both eyes are different (one brown and one blue, for example), while sectoral heterochromia is when one section of the iris is different from the rest. Central heterochromia is when the iris has a ring around it that's different. Though it can sometimes be a sign of an injury or other condition, heterochromia is most often a harmless — and cool-looking — genetic anomaly. | 
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