Ponyoria's Secret Feat. Sunset Shimmer

This is some nice clean lineart you have here.
You made a good use of your colors, even though there are some issues, but i will cover that later.
Also the illusion of an magazine cover is quite good.

What i noticed first was the huge resolution you use!
A high resolution becomes less relevant, the less details you show, but i see no reason for using a resolution of this caliber.

Also she looks a bit... streched?
If you put her chest, waist and crotch just half a head higher it would make a much more commensurate bodyshape.

I for myself would choose way thinner and shorter eyebrows, to create a more feminine look.

And the most important part...
Do not use grey, black or white tones for highlighting and shadowing!
The outcome will allways seem muddy and unnatural.
Allways use just a darker tone than your main color.
Use the CMYK sliders to create vibrant colors.

Also, do not use the airbrush tool for the forementioned tasks...
It will allways look synthetic, sloppy and like i said before, unnatural.

But i see that you are on a good way and i will look forward to see your improvement!