I went looking for an explanation because I wondered the same thing. I think that they just "highlighted" the planet for us (kind of poorly I think - they should have done it only from the perspective of the angle) because it wasn't as reflective as the rings and would be hard to see in "true color".
But I found an interesting video about that shot.
YouTube explanation video
My original post didn't work right - still things to learn about this forum I guess. I'm on a roll today, second attempt. heh