The red headed heroine with the flashing emerald eyes and temper to match is so common, you might never realize that red hair is natural to only about 1% of the total population. (Higher for those of Irish and Scottish descent, like in the 35 - 45%). Red is a recessive trait, which means it requires two parents with it (the trait, not necessarily manifest red hair) to produce a red headed child.
And interestingly, while blonde hair and blue eyes are genetically linked, eye colour is not linked to red hair. However, the majority of redheads have brown/hazel eyes, not green eyes. Blue, being a recessive trait, means that blue-eyed red heads are the rarest of all.
So what, right. But it's character research. There was talk on a forum recently about how most romance leads are either dark or blonde. Think about that for a minute. I didn't get involved, because I'm already so good at pissing people off. But as it happens, I'd already decided to give a future male lead some glorious auburn or strawberry blonde curls, and I so wanted to avoid the red hair-green eyes cliche. Voila, research. And a post is born. And a character named Erik with dark hazel eyes and copper curls across his brow will soon drag an unsuspecting stranger to his family reunion. Or maybe his sister's engagement party.
Also, I'm dying my hair.