@akkiraI just shopped the drawing. If I remember it right the guy said he drew them with a graphite pencil.
I just took the airbrush, set the opacity around 30-45%, set the color, and then color the certain parts.
That's the thing, "graphite pencil"(sorry, didn't know its equivalent in English).
Its actually quite good, aside from the edges around the neck, it really looks like a genuine colouring, it initially fooled me, I tried to get the same effect with that style(in something else, unrelated to ponies, more like a job thing) but it didn't turn out to be properly, vectors do not difuminate just as well unless some serious tweaking is done to the filters, seems a hell of a lot easier on gimp, but I haven't used image modifications tools properly in almost 2 years, the whole interface is quite confusing(CS4 is the one I got used to), and I didn't wanted to vectorize or redraw the thing.
And that's it, but before leaving.
Thanks, actually there was more down to her, but it wasn't Celestia anymore so I decided to keep it brief, and Celestia wise. Maybe a Luna will follow, but Nightmare moon doesn't provides as much as I hope.
I really need this sleep.
Goodnight everyone.