Photoshopping an eye to look like a natural fantasy eye picture can be easy through the use of masking, layering, quick-selecting, and color/hue changes. I have two examples here, including the good eye and evil eye. Below are the after/before pictures of the good eye and the evil eye.  Keep in mind that you can also do things like skin color change, etc. through the use of the quick selection tool and masking. I used those particular tools especially for the evil eyes color changes. 

For the good eye, masking out the eyeball and changing the tone of skin to a lighter tone makes it more angelic. Most of the work here is done through color balance, but to make the color changes simpler, making the eyeball black and white made the eye color faster to work with. 

The evil eye's color changes were already set in the same manner as the good eye above, so the layers aren't shown here. However, what was done in these steps was to put the oval vertical yellow shape in the center of the eye, and then overlay it with a copied layer of the background with the quick selection tool in layer 1 (masked out the center), setting the opacity of the layer to 64 percent. To make the oval shape seem natural and eerie, I also applied a small blur to it.