I think this is a very successful photograph. The subject is very involving with her open expression and enough information (towel, straps of swimming suit, slightly damp hair) for the viewer to tell that she's just been swimming and might be a little chilled. The color of the towel works perfectly in the environment and the contrasting color of the sunset is very nice. The overall pallet, while cool, doesn't feel cold possibly because of the dominance of purple rather than blue - which also gives it a rich, full feeling. The out of focus background, of course, calls attention to the foreground but it also emphasizes the fold in the towel and the strands of hair over her face.

Compositionally the elements reinforce the point of the photograph - the subject's face. The central vertical folds of the towel return the viewer to her face as do the horizon and the tree branches - in fact the tree branches seem to be reaching down to touch her.

Hope that helps. Best of luck!