Rainbows have always been a favorite of mine, especially with the way that the light diffracts and blends together softly. I like the soft lines and the gentle transitions between colors, as it's very close to and reminiscent of the real thing. This, I think, is improved by the layering of the border and the smudgy paint in the background. The fairy, I wish we could see more of... perhaps with a leaf crown, translucent wings, and sparkles. Sparkles are awesome.
Overall, I think it is very colorful and vibrant, mildly poetic, nice artistic technique. Lovely work as always, chica!