Yes, I sketch first to learn what to do with the elements I want to draw. Anyhow, little things might change during drawing.It look fine, though I wouldn't call it an illustration. It needs a bit of refinement as before, such as the butterflies/moths and the tree branch etc.
Rainbows are difficult in that style, but it needs to be a more definite curve and more indistinct - it's too strong compared to the moon, for example.
I would work on your composition too, do you do a detailed sketch first?