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Select your Gradient tool, then go to gradient options to make your own gradient. Select the "Foreground to Background" gradient. Now add a white color right in the middle of it. Change the other two (left and right colors) to light blue. Click here to see how your gradient shood look like. Click OK when done. Fill your selection with your gradient by dragging from top of the selection to the bottom. Deselect your selection when done.
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Use the same technique as for the main window to fill the rest of your white spaces with gradient. Don't forget to contract the selection by 2px, before you fill them up, or it will overlap the beveled borders. Also make sure you make a new layer for everyone of them if you plan to make future adjustments.
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To Colorize your skin, select your main skin layer's transparency (selection) by Ctrl+Clicking (Mac: Command+Clicking) on it. Make a new layer just above your main skin layer and then fill your selection with light blue color or any other color you want. From the Layer Mode combobox choose Color. Deselect your selection.
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In this final step I have added some more details to the skin, they're very basic. You may further modify your skin by trying to apply some of the styles to it and texturizing it. Whew....a long one! :))
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