1. Open your image or drawing, here I'll be using the lovely eye of Sarah Michelle Gellar better known as Buffy The Vampire Slayer
2. Zoom in on the eye you want to change untill you have a comfortable work area. Create a new layer (Shift+Ctrl+N), select the Elliptical Marquee Tool and draw an ellips around the pupil.
If you have trouble drawing the ellips you can always enlarge or size down your ellips by clicking Select > Modify > Expand for making it bigger or Select > Modify > Contract for making it smaller. 1 or 2 pixels should do the trick.
Usefull: hold down the left mouse button and the spacebar if you want to position your circle
Don't worry if your ellips covers up the eye lashes a bit, use the eraser tool to clean it up at the end of the tutorial
3. Choose a color and fill the layer with the Paint Bucket Tool. For example use #FF021A and change the blending mode in the layers palette to Overlay. This is what you get:
4. Don't like the color ?? Choose another color or for more detailed options go to Image > Adjustment> Hue/Saturation (CTRL+U), check the Colorize box and freestyle with the options untill you get the desired effect
More Tutorials: 
