This "birds of a feather flock together" attitude, would appear to be in part what creates and maintains diverse species and biological diversity in nature, which insures diversity and the survival of that particular genus or species. If this wasn't the case, wouldn't we all just be one celled organisms, who evolved the same, and looked exactly the same? Where's the biological diversity to insure every species needed to maintain ecological ballence would survive to fill those necessary ecological rolls? A genus will typically play a very important roll in creating environmental ballence to sustain a healthy environment, not to mention have an important roll in holding up the food chain/web.
That being said, even birds have a "pecking" order, so maybe you just have a sensitive or unlearned duck regarding appropriate duck edict. I've often observed flocks treating one duck with less regard, but in careful observation, I've never seen it relate to color. Rather it appears more relevant to the individuality of a particular duck like age, size, temperament and personality as the relate to the flock dynamic.