J. Glick submits this question to the Emergency room of Photoshop 911:
"Dear PS 911,
How is the best way to change the color of a person's eyes?"
Changing Eye Color...
This one is pretty simple, and there are several different methods you can use.
1) Replace Color
2) Hue/Saturation - Colorize
... are my favorites...
REPLACING COLOR METHOD
This has been around since the dawn of Photoshop, I'm surprised more people don't use the Replace Color tools... they're very powerful and easy to use. They're also highly useful in a number of scenarios where you wish to change the color of an object.
This one is more accurate so I'll list it first. (Photoshop 7)
* Choose Image > Adjustments > Replace Color
This brings up the Replace Color Dialog with a neat little Mask thumbnail.
Select the color you want to replace in the image window
Click the "Selection" button to view the image, or click the "Image" button to see the entire image.
Use the Eyedropper with the Plus symbol to "ADD" color to your selection.
As you ad, notice the mask adds that color. Be careful not to add too much color.
When satisfied with the selection, adjust the hue, saturation and lightness sliders until done.
Don't forget you can also use the "Minus" Eyedropper to remove specific colors from the selection.
In Photoshop cs it's been made easier through the use of the replace color tool.
Peek under the Healing brush Tool (tap j) and you'll find it.
It's a good idea to zoom in closely on the subject.
Now, open a file with the desired eye color and hold the Option Key (Mac) Alt Key (PC) with the tool to "sample" the color.
Return to the eyes to be colorized and paint in the new color. (Keep an eye on the options bar for ways to fine-tune your tool!"
ADJUSTMENT LAYER > HUE / SATURATION
This is the easiest but not quite as accurate.
Use the Lasso Tool to make a selection around the colored areas of the eye you want to change. (or any object for that matter -- also works well with Red Eye)
Choose Layers > New Adjustment Layer > Hue/Saturation
Click the "Colorize" button on the palette
Slide the Hue, Saturation and Lightness sliders until satisfied.