Blue Sky by Audrey Wood is a simple concept, but colorfully executed. Each two-page spread shows a beautiful crayon or pastel picture that depicts the type of sky it is. My favorite is "Sunset Sky", that is all yellow and oranges. Check out Audrey Wood's Website for more information on her and her husband and son.
I would use this with a Storytime on weather, or just Sky. You could easily incorporate it in a Colors theme or even Night Sky.