The reason we kept the blue-green is because, frankly, it looked pretty derpy going from red to green or orange to green. Additionally, Ahri's new look already has a ton of yellow fire on it, and the orb became hard to see against her most of the time. it's her defining characteristic, and not something we wanted becoming invisible against her fiery tails.
Those wondering why we are attached to the green hue -- it's because it intuitively connotates healing, spellvamp in this case. Ahri's new skin was a tough point of discussion -- we became faced with situations where we'd be changing things for the sake of change, while weakening the skin aesthetically and in terms of readability.