In R. A. Salvatore's books The Legend of Drizzt series, the main character Drizzt Do'Urden is a Drow, a species in Forgotten Realms that live underground and far cousins to elves. Drow have characteristically red eyes. However, Salvatore decided to give Drizzt purple eyes. What was behind the decision to give Drizzt purple eyes instead of the standard red eyes for Drow?
There appears to be very little behind the colour difference. Salvatore thought up a misfit drow character on the spur of a moment while very little was defined on the drow race. Drizzt was ahead of the actual setting.
Novelist R.A. Salvatore crosses over into his newest magical realm and brings along his favorite character (SciFi.com, archived by the Wayback Machine):
The editor called me at work and said, "I've got to go to a marketing meeting to sell the book, and we can't use [that character], and I really need to be able to tell marketing." I said, "Give me a little while. I'll call you back." She said, "Oh, no. You don't understand. I'm two minutes away from where I have to be five minutes ago." And off the top of my head I said a Dark Elf. And there was this long pause. "Yeah, a Dark Elf Ranger, that's cool. Nobody's done that." And there was a long pause, and she said, "There's a reason why no one's done that." I said, "No. It will be all right. It will be all right. It's just a sidekick character." And another pause. "What's his name?" And off the top of my head I said, "Drizzt Do'Urden?" Then a long pause. "Can you spell it?" I said, "Not a chance." It came to me off the top of my head.
The character's gotten more popular as time's gone on. After the third book, around 1990, people really wanted to know where this guy came from, and so I wrote the Dark Elf trilogy, a prequel to the other three books. And in doing that I created this society of Dark Elves, the Drow, in Forgotten Realms. And he kept going like the Energizer bunny. It's amazing to me. I don't know how. I don't know why. I try not to ask too many questions. [Laughs]
For continuity reasons alone the eye colour never changed. Salvatore never commented on it leaving the field to fan conjecture and trope theories. These vary from possible human ancestry to logical reasoning setting Drizzt apart from his red-eyed race:
The Legend of Drizzt on TV Tropes.
Drizzt's lavender eyes are remarked upon by anyone who sees it and are a pretty unique identifier as most drow have red eyes. They symbolically represent his more gentle and heroic nature compared to the rest of his brethren.
The Dark Elf Trilogy on TV Tropes:
Drizzt has Lavender Eyes, an unusual color for drow to the extent that his family initially wonders if he's blind. Has some Innocent Blue Eyes thrown in, since they come from combining a blue tint with the regular red glow from infravision.
Apparently drow eye colours became more varied eventually, as the setting grew over the years. Reasons for this were equally colourful.
Some more background:
I was wondering if there were a reason behind it, that's why i'm here; but since there is not, other than what Salvatore decided in the hurry, the simple answer is:
A genetic mutation. In real life brown is the color for human eyes and from somewhere (don't where and it doesn't really matter) humans have gotten blue eyes and the mix with blue and brown has caused a genetic mutation for the offspring to have green eyes. Maybe Drizzt was the first to have a genetic mutation of having lavender eyes, like a human would randomly get blue eyes instead of brown eyes. Also, there is a very rare eye color for humans, purple. What causes it? I don't know.
More magical approach for the reason for Drizzt's eyes is because he is a good person and not chaotic and evil. Much like enlightened good monks have blue, evil monks have red and neutral ones have purple, which is the mix of two. This does let me to think Drizzt would be neutral, but no, Drizzt is definitely a very kind person. So let's just scrap the monk theory, he after all is a ranger.
One last possibility could be that since Underdark is the size of the planet (afaik) there are likely to be more dark elf societies other than just the ones in Menzoberranzan, this means that while Menzoberranzan people have red eyes, some other dark elves from other societies could have other eye colors (like there now are), so it's possible that somewhere in Drizzt's family line, there has been someone from somewhere else, who didn't have red eyes, possibly purple and then the child got red eyes, but generations pass and Drizzt inherits his great grandfather's purple eyes.
But these theories are just that, theories, in no way canon, BUT based on human genetical facts. But Forgotten Realms is a place of magic, the real reason might as well be "Poof, magic!" We don't know the reason, but i hope my theories sheds some light on the matter and states that, even though we don't know the reason, there IS a reason (possibly, and hopefully :D, one of my theories) and not just utter non-sense.