A softer blue would be better, but I want to say that is going to be a wonderful project. Keep going and keep us posted.

Give me a brush and paints and I can paint a sky...LOL Although that is another option to consinder. You would need the fabric painting medium. You could dry brush darker colors for the dimension.
I made a denim shirt for halloween painting the trees, sky, and foreground. Then I embroidered a haunted house...fence with pumpkins, tombstones and some ghosts. Turned out really cute and it is completely washable.
This quilting is new to me. I am having so much fun though seeing what others are working on.
I am wondering if quilting with varigated thread would add the dark and light dimension? Or maybe quilting over a darker organza. The only FM quilting I have done is stippling. I have seen so many beautiful scenes that people free motion trees..flowers...etc.
I am working on my first quilt w/o being paper pieced. A bargello heart. I love the paper piecing because you never have to worry if you are cutting your pieces straight.

The pale blue would work, do you have some white fabric to make some clouds out of? Maybe a few wispy ones in the sky to add some interest?