Follow the same steps as Howto add the effect of a graduated neutral density filter on a photo using the Gimp graphics editor, except when you adjust the brightness, you can adjust the hue, or remove certain colors. In fact you can use any feature of Gimp on the graduated layer. You could also use a different method other than gradient.
Common graduated color filters
More intense skies
- Tobacco(very good)
- Of course these filter can and are used for things other than sky.