My impression on the GSD and the white color issue in that breed, is that the history of the breed involves its war dog history and other uses that were promoted by von Stephanitz in order to ensure the breed's development and survival. A dog that could do it all (and truly, is a breed that does).

Out of that would come prejudice over different physical attributes -- and depending on one's point of reference during the formation period, the reasons could be realistic or made to sound mysteriously complicated to the point of urban legend. ...and thus it is for 'white color' in GSDs.

Some traits were easier to control than others. A rough coat, poor earset or white color were among the visible things that were not favored. Color as we know, is always an easy focus point.

Try to be stealthy on enemy territory while using white wardogs or flashily marked ones. Try to secure an after-hours area (civilian or military) with the same colors which may potentially be easily seen and shot. . . Consider that finances and training take a lot of resources, so consistency in the replacement dogs helped increase their longevity in use. (Of course, we know modern personal protection dogs of other breeds, particularly for civilians, can be excepted for their colors as they are not as likely to be used in stealth work -- and many didn't have the same promotional history as the GSD.)

It was the development of the versatility of the breed that helped emphasize the dark dog model. That's my interpretation of the issues.