If he is large than he is probably a Rouen. Mallards are smaller and only have one stripe across their eyes. Rouens will have a second stripe under their eye.
Ours are half mallard and half rouen. Our ladies are significantly smaller than our males were, the males remained the same size as our...
Males change their coloring twice a year. They get their male colors in the fall to impress the ladies and they change again in spring to blend in when the ladies are laying. So that coloring isn't exclusive to females, it helps them blend in and raise young...
Our mallard males had female coloring and drake feathers for a while until they came into their mature colors.
Is she quacking or croaking?
Edit to add: I just looked at your pictures again and it looks like her tail is darkening and her underbelly is starting to fade to gray. That's what...