We're not sure why the coloration would make the swans more survivable. But, we do note that most colorful birds (such as songbirds, herons, etc.,) start out light gray or white in color and then get their actual colored plumage later.

In the case of white birds (such as egrets, ibis, swans, etc.) they usually start out dark in color and then become white in color. White cygnets are a leucistic coloration and therefore, not the usual "normal" recognized coloring of cygnets.

We also note that swans will continue to raise their young and spend time with the young as long as they are dark in color. Once juvenile swans get their orange bills or start to turn white, the parents disown the birds and chase them from the nesting area. At this point in time, the young swans are seen as rivals for the habitat area and are chased to form alliances with other juvenile swans.

You might want to investigate other species to see if the difference in plumage color from young to adult has been explained. Great question. Good luck. The Regal Swan