Simple yet Effective Technique in Photoshop
Hope you all know Adobe Photoshop very well. This tutorial is only for beginners for Photoshop.
We are going to discuss about a very very simple technique in photo-editing using Photoshop. We are oing to learn using Hue/Saturation in Photoshop. This is very effective if you want to change the color of any part of an image.
Using Hue/Saturation, we are going to change the color of retina of an eye. Once we are done with it, you will know how effective this option is in Photoshop.
First download a nice image of an eye, something like this...

Once you have image in Photoshop, select Magnetic Lasso tool or press L to select that tool. Using this tool, carefully select the retina of the eye making a selection.

In order to make the selection smooth, apply a feather of 3px to the selection by going to Slect->Modify->Feather.
After applying Feather to selection, we are ready to change the color of the selected area.
Goto Image->Adjustments->Hue/Saturation or Ctrl+U. Following dialogue will appear

First check the Colorize checkbox.
Now move the Hue slider to your desired color and check the output. The selected area will be colorized as per the Hue settings. Isn't it wonderful....
I love blue eyes, so this is what i made...You can color as per your requirement.

This is the mostly used and effective option in Adobe Photoshop, which can create wonders with your imagination.