Sky's color can add more feeling to an image and thus, to a book's cover too.
Let's see an easy and quick way to change the color:
First: Open your image in Paint.Net. Then go to Layers > Add New Layer and at the new layer's properties, set "Overlay".
Now go to Tools > Paint Bucket and at Colors, pick any color you wish. I used a Pink shade here. Then at Colors click on "More>>" and change Opacity - Alpha to 84. Then click on the new layer to color it.
If you don't like the shade you chose, go to Adjustment > Hue/Saturation and recolor it.
You can go wild with colors:
If you leave Opacity-Alpha at the maximum (255) when you pick the color, you can get amazing effects:
Black:
blue-ish:
