I painted this a few years ago with my art class as a refresher on drawing and values.  It's was primarily to emphasise the dark and light sides of an object.  What makes this piece interesting is that one side is a maroon and the other is a light pink, yet the watermelon is essentially the same substance and color on both sides.  The light and darkness contrast is something my art teacher regularly emphasizes as one of the essential things to a good painting.