I wonder why she is sitting in the dark. Maybe she can't sleep and decided to get some work done but didn't want to risk waking sweetie belle with light spilling in under her bedroom door or something.
I meant to comment on and favorite this ages ago, when I first saw it, but for whatever reason never got around to it then. In any case though I absolutely adore this picture; it's no secret that Rarity at work is one of my favorite things ever, and this has to be one of the best (if not the best) interpretations of that I've seen to date. I absolutely love your coloring style!
Amazing work on the shading and coloring work. Wish I knew my way around a tablet like that... My only concern would be that her front left leg looks a lot longer than her right, but overall this is a beautiful piece, and the choice of color scheme and strong contrast between the foreground and background do an amazing job of capturing Rarity's personality.