If you specialize in fantasy and/or paranormal covers then you know elements like magic or paranormal abilities will of course pop up in character design. You'll find witches, vampires, fairies, werewolves, and whatever else an author can imagine. There are times when these characters need a set of mystical, glowing eyes. 

Much like coloring hair, it's sometimes difficult to get them to look realistic. And by realistic I mean as if it's their natural eye color and not a painted layer you slapped over their eyes. 

There are designers who use a painting technique, it's takes time to perfect and understand how to do it without it looking ridiculous. It takes time and work, as do most things. The easiest way to color eyes if you are still learning to paint those eyeballs, is to find an overlay file of some sort. 

Let's take a look at things

I saw a designer turn in a graphic that looked like this for a contest. 

1) the coloring doesn't even cover the entire eye

2) while a person can be born with two different color eyes (Heterochromia), I have never met anyone who's eye is distinctly half one color and half another. Though if you're deal with a paranormal element I guess anything is possible, but let's at least make an attempt at making it appear natural

You can either do a Google search for eye overlays or check DeviantArt. It makes it easy to change eye color if you aren't practiced at it. I have a PSD file for eyes, though I can do paint (I've been practicing!) there are times when you need more than a simply color change. You'll notice in the image below the file also has dragon's eyes and cat's eyes, which have come in handy on several occasions.

You can mess with these files to get the desired look. Mess with hue and saturation, color balance and the blending mode to make your eyes as natural as you can.

For this example I used the above file to change the eyes. I played with the hue to make the green brighter, then overlaid it on the image and adjusted the size so it fit her eye. I used a layer mask to erase anything that needed to go, I often with erase the pupil because it will appear odd at times when you overlay the image on the eye. 

You can see here the eye with the arrow is not set to overlay and it looks less natural, there's no definition in her eye and it's sort of glassy looking. You can try different blend mode to achieve the look you're going for. The model in the image has light gray eyes so overlay worked well to blend the green in. If the eyes on the model are darker it can be difficult to get the eye color to change and look natural.

Now we want to give her eyes a little magic glow. If you think about how an animal's eyes appear at night, you know their entire eyes will glow when reflecting light. So to achieve a decent glow I will add both an inner glow and out glow to the eye layers.

This image has only the inner glow, which is extremely subtle

This image has only the outer glow which makes her eyes appear a little off and slightly creepy.

Let's throw in an example of the painting technique cause I feel like it. We'll use the same base image and this time I'll make her eyes red. The first thing I did was create two new layers. I do two separate layers because there are times when you'll end up having to set each eye to a different blending option simply because of angles and lighting one setting may work for one eye but not the other. It's easier to begin with two separate layers so you don't have to start over.

Next I picked a red and painted over her eyes. As it stands right now she looks fake and creepy. We definitely don't want to leave it like this. Generally you want to go with a darker color than the final result you're looking for because the color tends to become lighter when you change the blend mode.

I set both red layers to overlay (overlay usually works the best) the color is pretty but I want to go for a richer shade of red

I duplicated the layers and changed the blend mode to linear light and set the opacity to 30%

I duplicated the layers once more and set them to vivid light at 15% opacity and I added the glow to this layer to keep it subtle since the layers were only at 15%

As with a lot of things in design, there are multiple ways to do this find whatever works best for you and run with!