I'm not sure. I am not familiar with whites. But I have seen some poorly bred black and tans look very similar. I love it when people can say without a doubt that a dog is mixed without seeing a picture of the dog standing up.
Also, with all the variations of this breed, due to what they were bred for and where, and then the mixtures between the lines of this breed, it is a breed that has a LOT of variation.
If someone is telling you the dog is purebred, and has the sire and dam, and they look purebred, then you have to decide whether or not it is possible. And even then, sometimes a pup looks more like a grand sire than either parent.
I'm pretty sure he isn't pure, I got him from a pound. They said he was white GSD, but he has to be mixed with something cause he has yellow along his back and a wider snout. I thought he'd be mix with lab but idk now