You get a fair bit of misguided references on the web as well whuch unfortunately people believe & quote. Its a bit like the media & the old saying "No news is good unless its bad news" people tend to believe the worst & bad news makes the headlines When you think of other sheep herding breeds & realise how many are white you realise how flawed some of the arguments/writing is/are.
I did wonder about that too VB, ref the Pyranean Mountain dog but then again I do not know if they just protect the sheep or whether they ever herd, or whether it would even make a difference ref which job they have........
Whites can be registered & you could show one although it would not come anywhere unless it was at a white & longcoat show put on by the white & longcoat society. However you could be asked to leave a show class at a normal show if you have a GSD with no undercoat. As for longcoats they now sometimes have their own classes at breed shows & in Germany they are not stopped from showing. However they still have to have an undercoat. The problem with a white is it would not stand out when herding & the sheep may regard it differently. THere are sheep dogs who are white but they tend to be guarders like the marremma not herder guarders like the GSD. I would imagine when you have a dog whose job is to constantly circle the flock & keep them together while they are on the move it would be difficult if your dog was no different in colour from the sheep.