No. He's not a mix, and yes, I would know. It's my dog and I raised, showed, and worked GSD's in SAR for nearly 20 years.. In all honesty, the photo makes this mask a lot greyer than it is in person.
Also, I recommend looking up sables in GSD's. There are many other colors out there than the three you mention.
Because I'm the owner of the mask and the dog. Just happened to be cruising through to get eddie's link for someone, and noticed your comment.
Most welcome, glad to see you did!
Ah, never mind, someone else already mentioned that and you responded below!
Glad to see it's all good, sorry about that!