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For this tutorial you will need the gimp from http://www.gimp.org. We're going to change the iris color on an existing texture in as few steps as possible.
Load the image you want to modify into the gimp. It will usually be a jpeg image.
Go to the layers dialog and select duplicate layer.
Click on the background color in the Gimp toolbox twice and set the sliders for your new base eye color
In the layers dialog make sure you have the bottom layer selected. Hit Ctrl+A (Select|All) Ctrl+K (Edit|Clear) and Ctrl+Shift+A (Select|None) to fill the entire layer with your new background color.
Make sure that your top layer is selected.
Use the color picker tool to select the lightest (or close to it) color from the iris.
Using the wand tool, select the iris.
You will have to hold down the Shift key and click in the iris a few times to get it all selected.
This is where the real magic happens
Click on the foreground color in the toolbox (the one you saved 2 steps ago.) Select the text in the Html Notation box and copy it to the clipboard
Select Filters|Color|Color To Alpha from the menu in the picture edit window and you will get a dialog box like the one above. Click on the Color Select (outlined in red, ) paste the foreground color text into the HTML Notation box and click Ok.
Click OK in the Color to Alpha dialog and watch your eye color change.
Hit Ctrl+Shift+A (Select|None) and save your new image as a jpeg somewhere where you can find it. Click “Export” if a dialog comes up saying that JPEG can't handle transparency. Apply your new texture to your model and you're done.