I really love the idea, and the way that the rainbow trail fades- the dripping effect is very nicely done and is my favourite part of the picture. Dash herself looks amazing, I love how she’s shaded and her wings look awesome.
I like the textures too, and I can see what you tried to do with them, but they don't seem to fit in with the way you drew Dash. Don't get me wrong, she's brilliantly drawn, but she just looks out of place. The paper texture and shadows make her look flat, and almost like she's lying down on the corkboard instead of flying. To improve this, perhaps use a different background or remove/soften the shadows.
Her mane is also moving in the wrong direction, and would make a lot more sense if it was flowing upward, to match her flight pattern, than sideways. It would also make it more apparent that she is moving.
(This is my first critique so I'm so sorry if it's harsh ; _ ; )
Well done! I went through your gallery to see if you have improved as far as techniques. And I say that you have. The flow of the rainbow itself has a very sleek and smooth transition. The fading and the symmetry is right on. But as far as "rainbow dash" goes, she is slightly off compared to the flow and direction you have the picture going. I noticed that you are considering gravity when it comes to the start of the rainbow, but dash is reversed. First of all, her main seems to be flowing the wrong direction and the curve of her body is too sharp compared to the rainbow's curve. Other than that, the coloring and shadows placed on dash are perfect. I like the cork board you've placed as your backdrop. I myself Enjoy the earth tone type contrasts. Altogether, this is a beautiful photo that shows a new retro feel to the character "Rainbow Dash." Thank you very much for putting time and effort into the world of art! ~Aikira-chan