I believe that the dark background has a purpose, it has been used elsewhere and is needed for the look of the bubbles to be effective, which are the focus of the challenge itself.
EDIT: Sorta kinda fixed the major things
EDIT2: Cutified nose, face in general, stray white pix, aa. Scrapped the grey highlighting on the hair; Doesn't help to make her look any younger
Super update: Pastelised colours, redid head (esp angle) Thanks, Ska!
The original "finished" version can be found here
Edit3: Changed some stuff up, mostly chest area and arm- Thanks Lollige