From what I've researched you never can tell if you'll get a PURE white snake. However everything I've seen points to Butter x Butter or Lesser x Lesser OR Butter x Lesser BELs being pure white or close to it while Mojo x Mojo crosses are a bit gray around the head.
Mojo x Lesser/Butter crosses are suppossed to be pretty white too as the Butter/Lesser cleans up the Mojo grey.