this is the most important thing:

"The 'sky' of a world refers to the view of the heavens from its surface. This view varies from world to world for many reasons. The most important factor in the appearance of a world's sky is its atmosphere, or the lack thereof. Depending on the atmosphere's density and chemical composition, a world's sky may be any number of colors. Clouds may or may not be present and they may also be noticeably colored. Another factor is the astronomical objects that may appear in a world's sky, such as the Sun, stars, moons, planets, and rings."