This will be our starting point.
Select Edit in Quick Mask Mode. And We will begin painting the eyes to make our selection.
Once the eyes have been painted to your satisfaction choose Select from the main menu, the choose Inverse.
From the layers menu once you have Inversed your selection, create a new Adjustment Layer.
Select Color Balance, and begin altering the eye color of the subject.
In the end your subject should have a new eye color that looks fairly natural. Like so:
And thats it your done!
But if you want to use the techniques from my Model Glowing Skin Tutorial, you can have something like this: A subject with striking eyes and smooth skin.