What would cause a mallard have different colours? he almost an albino with the marking but in brown, tawny and white body and a grey head all in the same places as a drake. He grew up with a group of mallard his feet are very light orange.
A lot of the weird and wonderful looking "mallard" types have been developed for domestic use from the original wild Mallard. Even the white domestic farm duck originated from the wild Mallard.
Take a look at the Manky Mallards and you will see what I mean.