Adding new colour variants to an exisiting model is usually considered something of a copout from smartphone manufacturers, a sly way to repackage something old with a fresh coat of exclusive paint to move more units.
In the case of OnePlus and their SandStone White Limited Edition there's a complex paint and treatment process that adds hours to make the device in order to give it that cool matte texture and white colour. OnePlue is selling the 5T for the same price as the regular coloured models but possibly in limited quantities. Orders will begin on January 9th.