Saying "my character has purple eyes" will get people telling you to change them without any further information. You can either blame Mary Sue or the Drow species.
Despite this automatic shudder when presented with unusual eyes, it's not a bad thing to have a physically distinctive character. Common fantasy tropes are common because people like them. People like characters who look different, when it's handled well.
One way to remove the gimmicky feel is to portray the unusual eyes realistically. This post starts by looking at some uncommon real eye colours and has a few thoughts on inventing your own.
Real Eye Colours
Green Eyes - Green eyes are the rarest common eye colour, and thus still get unusual eye colour status. They're based entirely on pigments (which sets them apart from most of the others discussed in this section). It's odd that novels link green eyes to red hair and pale features, because green eyes found in many parts of the world and in people with all hair colours. The trick with green eye realism is to shake up the stereotypes - don't just give them to Irish people (and don't make all your Irish people green-eyed and red-headed).
Yellow Eyes - These are also within the range of normal eye colours (and would be classified as amber), but it's rare to see someone with completely yellow eyes. It's a surprise they aren't more common in fantasy books. Maybe yellow isn't deemed interesting enough?
Violet / Purple Eyes - Violet eyes are caused by blue eyes mixed in with blood colouration*. You'll most commonly see them described in connection with human albinos. Most human albinos have blue eyes. In cases where there's very little pigment, blood colouration may show through, thus making the eyes appear violet under normal lighting conditions. This lack of pigment often leads to poor vision.
It's not impossible to see violet eyes in a non-albino person. The one violet-eyed individual I met had damage/deformity in the outer eye, which meant there was blood colouration in the iris area. Again, red + blue = violet.
It's also within the realms of possibility that a non-albino may have albinistic eyes (reduced pigment only in the eye area). I couldn't find a confirmed case for humans, but it sounds realistic enough to include in a fantasy novel**. A person in this category would have the same vision problems as a full albino.
Pink Eyes - As said before, most human albinos have blue eyes. However, in some cases, there may be so little pigment that the eyes appear pink (from blood showing through). Given that pink-eyed human albinos are rare, this colour choice might make it look like you didn't do your research.
(Note that albinos often appear to have pink or red eyes in photographs due to the reflections. This doesn't mean they appear to have pink or red eyes normally.)
Black Eyes - Really black, not dark brown. This is a caused by a condition called aniridia, where the iris fails to develop (along with some other problems with eye development). This leads to the eye appearing to have an enlarged pupil directly on the white of the eye. People with aniridia will have vision problems.
On Inventing Pigments
Let's say you decide you want pink eyes, but you want it to be caused by a pigment. You've immediately made pink eyes more common. As well as a scattering of pink-eyed people, you'd expect to see people with odd flecks of pink in their eyes, mixed colour eyes, and all the combinations we see with normal eye pigments.
Consider that the rarest common eye colour is green... you wouldn't have a chosen one marked by their green eyes, unless it was a case that any green-eyed person would do***.
This may be exactly what you want. It could be fun to explore a human population with new eye colours. If it's not what you want, and you want an eye-colour people have rarely seen (or may never have heard of) you're better off with an eye colour linked to a medical condition.
No, Not Purple!
Having medical implications to eye colour, or making it a common eye colour in your population****, are two ways to make it realistic. Some other thoughts on reducing the 'argh, purple eyes!' reaction are:
- Eye colour is usually subtle and you have to get close to see it. Violet eyes tend to be greyish violet (much as blue eyes are often greyish blue). Noticeable, but not the first thing you notice about a person. It comes across as more realistic if eye colour is something observers notice later.
That said, if your character has neon green eyes, it'll be noticeable at a distance. They're probably glowing.
- Some of the 'argh, purple eyes!' response is a reaction to authors mentioning eye colour continuously. Don't get so caught up on the unusual eye colour that it's mentioned every five seconds.
- Why do unusual eyes have to be seen as pretty? They may appear ugly or freaky to some other characters. Some characters may be indifferent.
For Those Who Skipped to the End
Make your lavender-eyed princess short-sighted and give her glasses.
* Though I've seen claims of violet eyes due to a pigment, I haven't seen any evidence to back it up. All the cases I've heard of, with reasonable evidence to back them up, are due to blue pigmented eyes and blood colouration combining to make violet.
It is possible to make light blue or grey eyes appear purple with special lighting, clothes and makeup (which is possibly the origin of claims of a purple pigment).
** Cats have a number of genes causing white fur patches and/or blue eyes, so the idea of albinism being restricted to certain areas of the body isn't unheard of in nature.
*** Big Trouble in Little China took this route.
**** It'd be funny to have a population of people with dark purple eyes and make the unusual eye colour brown. If there isn't a fantasy parody using that idea already, I'm a mushroom.
(It shouldn't need to be said that the eye photo is me playing around with graphics programs, but I've seen people linking to similar photos as proof of dark purple eyes existing. So people googling purple eyes: they exist, but not like that.)