Blue/grey? That's different. I've seen the girls' bills go almost brown/black, with very little orange, and the boys go anywhere from yellow to green, but never blue. Males will start molting between 2 and 3 months, from my experience, and that's when they get their signature green head and tail curl. But their bill color changes just before that. Obviously with most ducks, voice-sexing is the first way to tell, but with your "oddball" I'm guessing you'll have to watch during the next few weeks and see if it starts changing colors. Below is my Dexter, when he was just a couple weeks older than your ducks, so can see what to watch for (he's the one in the middle in the pool pic)Another thing about the bill color, it seems to have a blue/grey hue to it. Shouldn't the tail feathers have a curl by now if it were a male? My mallard also lacks this.