Paul Newman wasn't too fond of his blue eyes. The actor, who earned admiration from audiences and critics in everything from Cool Hand Luke to The Verdict, had a piercing set of blue irises that were as recognizable as Sylvester Stallone’s deltoids. He found the attention they received slightly grating. “If blue eyes are what it’s all about … I may as well turn in my union card right now and go into gardening,” the actor/philanthropist told The Saturday Evening Post in 1968.

If Newman knew the science behind his distinctive peepers, maybe he wouldn’t have been so hard on them. Although your genes are responsible for the color of your eyes, it’s a very complicated hereditary trait. Where you fall on the spectrum from light Newman blue to dark brown is the result of how much melanin pigmentation you have.

The iris—the colored part of the eye surrounding the pupil—is made of layers. The iris pigment epithelium is in the back and has some black or brown pigmentation to it. The layer over it is the stroma, which sometimes has brown melanin pigment, as well as colorless collagen. The black dots or “strings” you see in the eye are typically coming from the epithelium and are visible through the stroma.

Color is determined by the amount of melanin in the stroma. If you have brown eyes, you have brown melanin in the stroma that will absorb available light and make the iris appear darker in color. If you have green eyes, there’s not much melanin or collagen, and the light will be scattered. If your eyes are blue, you have no melanin at all—all of the light hitting the eye is scattered and reflected back. That’s why people with blue or green eyes can seem to shift eye color, depending on the amount of light in a room.

So how are genes involved? While they don’t directly program your body for a certain eye color, they do affect the quality and quantity of melanin in the stroma, which dictates your hue. While Newman’s brand of blue is a little more unusual than brown—the most common color—he probably would’ve been equally perturbed by grey. That shade, possibly caused by excess collagen, is considered the rarest eye color of all.

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