Here is a very interesting tutorial on how to retouch, or let me say change, the eye ball in a portrait image using adobe photoshop. This tutorial was created by Chip Springer and all pics in this tutorial were also posted by chip.
This tutorial requires, in my opinion, a fairely advanced level of adobe photoshop knowledge, since I am still not able to obtain satisfactory results from it
Here is the eye template to be used in adobe photoshop
You might want to turn this template into an adobe photoshop brush.
Start with a solid color (you can always change it later) at 100% opacity covering the eye and move this template over it using the transform tool and a mask to fit.
Play with Levels to tweak template layer.
Add a pupil and catchlight.
Don’t forget to play with the layer modes on the template layer, also, use Levels on the solid
Here is another example rendering a multicolored eye
For more info about this tutorial, please refer to its original thread on dpreview