I will be using this photo of Miley Cyrus found at Heart-Miley.com.

1. Open your image in photoshop.
2. Select the polygon lasso tool , and ensure anti-alias is turned on and feather is set to 1px:
    
3. Zoom in on one of the eyes and carefully select the iris. Zoom in on the other eye, hold down the shift button and do the same on the other iris. 
4. Next, duplicate this selection
   (CTRL+C then CTRL+V)  
5. Then edit the colour varciations of this selection:
   (Image » Adjustments » Variations) 
6. Ensure your setting are as shown here:
    
7. Then simply click on the different colours until your Current Pick looks the way you want it to. Press OK when you are happy with the look.
8. Erase the colour over the pupil (black dot in the centre of the eye). Erase any messy parts around the iris if you need to. 
9. Save your picture, and you’re done!