This is a response for Lindi Burger's question to me on how I enhanced the color of the eyes in one of her shots.
Lindi, sorry it took me so long to get back with you...I've been a little busy.
The easiest way to do any selective modifications is to use a mask.
In the case of your shot, I used photoshop's quick mask to isolate the pupils then increase the color saturation. Below are the steps I took (with screen captures).
The very first thing you want to do is to duplicate the layer of the image you're working on. This gives you a clean copy to revert back to in case you really go too far with some changes and it would really be easier to start over than to undo the changes.
You can duplicate the layer a couple of ways but I'm used to just right clicking on the layer and selecting "Duplicate Layer" from the drop-down menu.
Next, ensure your Foreground and Background colors are reset to the defaults (black and white). These are the control colors Photoshop uses to create a mask and to erase a mask. Black makes the mask and White erases.
Just click on the default foreground/background icon (the small, overlapping squares with the black square overlapping the white square). The color of the square on top indicates the color of the foreground and the background color is the color underneath it.
Now you want to zoom in on the area you want to mask so you can get the masking as detailed as practical. You should zoom in at least 200%. Many times I zoom in to 400-500%.
Now you want to enter the "Quick Mask" mode. Just click on the quick mask icon (the rectangle with a circle in its center).
To mask any area of the image, you need to use the Brush tool so select it. and then modify the brush's properties (right-click anywhere on the image) so the "Hardness" is around 95%. You can also change the size of the brush at this step but you can't see the changes until you exit the properties box. To exit the properties box, I just click anywhere on the menu bar.
Now you should change the brush size so its smaller than the area you're masking. Easiest way is to use the left and right bracket keys. The left-bracket ( [ ) key decreases the brush size while the right-bracket ( ] ) key increased the brush size. I found 15 pixels was a good size for this application.
Don't worry if you mask an area you didn't want to mask. You can remove any portion for the mask before you finish. This is one of the benefits of using a mask--you can make mistakes without causing any problem before you execute the mask. To erase a mistake, just change the foreground color to white (that's why I had you reset the foreground/background colors before). To toggle the foreground color between black and while, just click on the large black square overlapping the white square. The black will toggle to white and the white will toggle to black. Now just use the brush took like before but paint over the mistakes and they'll vanish.
Once you're finished masking the areas you want, you need to exit the quick-mask mode. Do this by clicking on the quick-mask icon again (the rectangle with a circle centered in it).
Once you exit back to normal editing mode, the areas of the image which were NOT masked are selected. However, you don't want to select all but the eyes' pupils, you just want the pupils. To change the selection to just the pupils, you must "Inverse" the selection. From the "Select" menu, click on "Inverse" and the selected areas of the image will invert so only the areas you masked are selected.
Now you can make modification to the selected areas without affecting the rest of the image. In this case, I just increased the color saturation (Ctrl-U). In this example I'm increasing the Master color saturation by 66%. If you wanted, you can selectively increase any of the primary or secondary colors by selecting one of them from the drop-down list.
And there you have your finished image with enhanced eye color.
Hope this helps you out. Have fun working with masks!