This is a lot of fun and you can do it using the Color Balance method or the Selective Color:
COLOR BALANCE:
- From the Toolbox, click on the Elliptical Marquee Tool. Feather: 5 pixels (this can change depending on the photo that you are using).
- Draw a circle, making sure that you are getting inside the whole pupil.
- Press Ctrl+B to get the Color Balance dialog box.
- Click on the Midtones radio button and now play a little with the color sliders (Red, Green, Blue…) until you get the color of your preference.
- Now, click on the Shadows radio button to bring more life to the eye. Click OK.
SELECTIVE COLOR:
- Select the pupil/s as you did before.
- Go to Image, Adjustment, Selective Color.
- Select Neutrals from the Color drop down. On Method, click on the Absolute radio button.
- Now, you can change the amount of color moving the sliders for Cyan, Magenta, Yellow and Black, until you achieve the color of your preference.
- Click OK.