What breed are you referring to?
Sometimes it depends on the duck and breed, but yes both the bill and the feet can change color.
A few examples:
Khaki campbells: as they mature, males will sometimes get orange feet
Welsh Harlequins: when they hatch, females have light colored bills and males have gray bills. As they grow, all will turn gray. Once they start to mature, females will get gray bills (though some can stay light colored) and males will usually turn yellow.
Some will also develop "spots" on their bills, dark freckle-like dots.