I have a question about Mallard Ducks?

Last year i hand raised a mallard duck, i released him when i saw that his male colors were appearing, Its been around 6-7 months now but a few days ago a male duck and a female appeared at my sliding glass door eating my dogs food , I put out a bowl of meat builder i had leftover from last year and they gobbled it up with no hesitation...so im thinking this is my duck but my duck he had a solid grey beak, and this ducks beak is yellow ... So Do Ducks Beaks Change Color? and Do You Think This Is My Duck