I know you said you don't like this picture. Its good though. I think we all have that picture that we don't particularly like, but others seem to like. I can see how you don't like the clothing you did for her. Nothing wrong with the colours you used. Though the red skirt with the purple striped socks might be what sets it off a bit. Fashion is not always an easy thing to get across in a character. Though a lot of the time Equestria girls tends to make the outfits that their characters wear very...... colourful. So your not too far off with what Sonata might wear. Some times drawings don't come out exactly the way we want them to, or we struggle with a certain aspect, and that can be frustrating. Like all of your other works though, this is still very well done.