To regulate the about of light entering the eye and hitting the retina.
Just like the "iris" on a camera which controls the amount of light hitting the film, muscles controlling the eye's iris can contract, drawing it back away from the eye, or relax, causing the iris to contract over the eye.
The colour (blue, green or brown) is irrelevant and serves no further purpose beyond that of hair colour - i.e. it is a genetic mutation that makes one member of a species more or less attractive to others (depending on taste!) which is then acted on by the processes of natural selection.
What is the function of the iris?
It expands and contracts to allow more or less light onto the retina depending upon lighting conditions. It also comes in many colours - usually in pairs - and can be very attractive. Without them, a pretty green eyed blonde girl would just be a pretty blonde girl in sunglasses ;-)
soap opera
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