I have a White GSD and have also kept Black and Tans. I have found my White to be more alert and easier to train but also to be a lot more weary of strangers. He is happy for them to come into the house as long as it's on invitation. He would make a very good guard dog where as our last one, Duke, would have licked the burglar to death
I think that what I'm actually trying to day is that it isn't down to the colour, it's down to the individuals nature.
I think that Whites are more likely to end up in rescue due to the type of people who generally buy them, not because of the dogs temperament (which is often believed to be the case).