Pretty good work here again. I really like the intensity of those rainbow colors. I don't normally work with traditional media myself, so that's impressive to me.
I like the way you pulled off the shading for this one. Applejack looked too dark and colorless in places, and Rarity was fine because her coat is white anyway. But here, you seem to have found a pretty happy medium giving some decent sense of structure with the shading, while not taking away from the color of the coat.
Speaking of shading, I really like the way the wing casts its own shadow on her hind leg. Nice attention to detail. However, it seems to me she could use a little bit of shadow near the top of her head, towards the back, cast by her bangs. I might also suggest just throwing in a little bit more shading in the hair itself, and intensifying the highlights a little bit, to give us more a sense of the structure of her hair.
I like the pose. She looks like she's rearing back slightly, ready to launch herself forward, presumably because she's challenging the viewer to a race. It's done pretty well, but I might have gone a little bit more extreme with the angling of her legs and back, bringing her fore-hooves a bit more forward. It would have intensified her pose, and would have distinguished it a little better from Applejack's pose.
Overall, great job. Great addition to the series.
I know what you mean about the hair. I just remembered in bed last night that I forgot to shade under it and I also should have shaded the hair. I was so busy making different shades with my pencil for the rainbow colors that I didn't think about adding light and dark spots but I did make some reflective highlights...
THanks again for the critique.