There are many methods to do this, and some look better than others. This particular method is the more natural way to add color and contrast without appearing unrealistic or "photoshopped."
Step 1: Basic Retouching
Step 2: Selecting the Iris
Step 3: Adding Color
Using the brush tool, I'm going to set the opacity on 15% and put it on a color blend mode. This will allow me to paint the color into her eye without painting out the details. Using outward brush strokes, paint around the pupil to give it and uneven coloring that follows the natural lines in her eye.
Step 4: Preparing to Add Contrast
Step 5: Adding Contrast
Step 6: Final Touches
In order to blend the colors in a bit better, I went back to my color picker, (still using the brush on 15% and the color blend mode), and add more brown to the outside edges and a little bit into the green.
After blending the color, I added my final touches by dodging and burning parts in the iris, (remember, be sure to have your brush set to midtones with an opacity of 10-15%), such as some of the darker spots and details and the outer rim.
As I said before, there are many ways to do this, and each method has it's benefits. The results of this method may vary depending on the original photo and eye color of course, but I feel it is a simple proccess that works well.
If you're looking to make your eye pop even more, Here's an instructable on adding more "life" to eyes. :)