You cannot differentiate the gender of a cygnet by color, nor the fully grown swans. Mute Swans regardless of sex are white, sometimes with gray or buff tipping on the feathers. The best way to tell cob (male) from pen (female) is to look at the size of their beak bulb. The male will always have a larger black nodule on his beak. Size is also a factor; with the cob being noticably larger than the pen.
The only true way to know a waterfowl's gender (not withstanding the obvious species that have a color difference) is to have vent the bird. Venting should ONLY be performed by a professional.
Enjoy the swans!