Just seeing that snake I first thought white diamond or phantom lesser. Both tend to have a yellow dorsal. Looking at the parents, neither look like a russo, so is there a chance one of them is a phantom or other gene in the BEL complex? I'm not well versed with IDing most of them. I guess the test would be to breed to a super mojave, if just white snakes pop out, then guess you have the most yellowed out super lesser I have ever seen, if something else pops out, then guess it's another BEL gene.
Anyone saying white diamonds are the whitest BEL really makes me wonder if they have actually seen one... I've seen quite a few in real life and not a single one was pure white, all had a yellow dorsal of some extent. Also I have seen more yellowed out lesser/mojave than super lessers, I would personally say you still have a better chance with super lessers for pure white, but neither guarantees pure white.