Colour overlay the sky
Getting the right gradient between the lighting conditions can make the sky appear murkier or of a less prominent colour than required. To remedy this add a layer of a block colour, such as a deep red or blue, and drop the opacity to under 50%. This will give you a light hue within the sky of a particular layer, sitting between the desired layers and providing an effect that can emphasise a particular existing shade. This technique also works extremely well on a standard landscape image, remedying to some degree a lack of a slide-in graduated filter or a sky with complex tones to balance.