This tutorial is probably one of the easiest way to change eye color. First thing you need to do is to go to this link http://www.brushesdownload.com/bfile.asp?id=3113 and download the Contact Lense PS Brush. After downloading, here are the steps to follow.
STEP 1: Choose “Brush Tool”
STEP 2: “Right Click and load the Contact Lense PS Brush.
STEP 3: Create “New Layer”
STEP 4: Change MODE to “Soft light”.
STEP 5: Adjust Opacity.
STEP 6: Point Contact lense PS brush on eye lens.
STEP 7: Use “Eraser Tool” to remove excess brush.
Then it’s done!
Here is the result: