Domestic Ducks are usually white in color because they were and are bred with use of only other white ducks.
If a white duck and a white duck mate then the baby duck will usually turn out to be white in color.
However there still are some other domestic ducks that are different colors but that's also rare and most domestic ducks you see are white.
It's also because some domesticated breeds of duck were selectively bred to be that color of white.
But some duck breeders breed domestic ducks to be brown and other colors.
So you can still come across some domestic ducks that are brown or any other color although the most common color for domestic ducks are white.
If you look around for other duck breeders you're sure to eventually find other colors of domestic ducks.