Since the male is a 'Mallard', the female probably just turned out lighter colored than usual.
Or if they are 'tame ducks', she may be mixed with 'Pekin's' they are white.
I have had female Mallards the same color.
I made it out to a place around my house a couple of weeks ago where there were lots of ducks and geese. Here are 2 pics of the odd colored malards I saw.