Rainbow is highly detailed and has really nice bold colors. Especially with her hair. I also like that subtle grin on her face indicating her love of flying. Her expression and her pose also makes her look like she can do so effortlessly.
The way you made the background and clouds really make her stand out by giving them a "softer" feel, so to speak. They aren't blurry such as when you make bokeh with a camera lens, but are just of a different, more patchy style.
Speaking of the background, drawing it with a tilt and having her facing the ground give off the impression that she is doing a flip or some other aerial acrobatic move. A really nice touch!