The sky will dictate the mood of a painting and will dictate the colors of the painting.  The highlight colors of the painting and the reflected colors will be contained in the sky color. For example if you have a late evening sky such as in the photo below. The bright colors in your sky will be the brightest highlights on rocks. You won’t have white snow for instance.

One thing artists love to do is give the feeling of sunlight. However pigment is not capable of doing this. It is recommendable that you don’t depict the sun directly.  Somewhat conceal it  behind a mountain, tree, cloud, etc. That way you show the rays or just a peek of it. The exception of this would be a foggy day. I have seen paintings where the artists successfully show the round sun but the fog will soften the impact of the light.