Ok ready for some irony? If it is pastel and it is a good pastel it will most likely not fade. In any case if it is a pastel I don't think it will ever be a pure white. Super Mojaves are not pure white but they are not yellow either. If you were after a breeder this is a great animal. Even bred to a normal it would produce 100% mojaves with 50% being pastaves.
If it is hypo, it will fade. I am not really the guy to comment on hypo. I have never produced any. I was originally going to say no way it is hypo until I took a look at the hypos here. The baby pics were surprising at least to me.
In all cases this will never be a true white snake. The consistently whitest snake produced is a Lesser/Mojave or a Lesser/Lesser. Lesser/Lesser tend to have eye problems so I avoid that combo.