it appears to be a white duckling. Likely a recessive white gene hidden in those parent birds. Hatched white mallards once from a pair we had. Totally threw us for a loop for a while trying to remember what we had put into the incubator, since we had no pure white breeding birds at the time.
K&S
ohh thanks everyone, the whole the parents being a completly different colour confused me completly! i did think it could be white. its not so obvious in the pictures but it defiantly has a darker head, neck and back area so buff could be the one.