Probably a mistake but a very interesting one. I've seen many mistakes on Mufasa but none featured him with a mane of two different colors.
Maybe the artist had read the tale of two brother's book and decided since Ahadi's mane was shown as red but said as black, he though it was a good idea mix them both then make Mufasa look more like Ahadi.
http://www.amazon.com/gp/product/images/0736401237/sr=1-5/qid=1266199117/ref=dp_image_0?ie=UTF8&n=283155&s=books&qid=1266199117&sr=1-5
Also notice Scar looks Nuka-like.Very preculiar