Oh this is quite interesting Richie because we are going through the same thing! We've had mallards here on the pond nearly every day for the last 2 months and I'm hoping for ducklings!
We too normally have mr and mrs, or sometimes mr and 2 mrs's (how do you write that?

). Sometimes there's another mr and I've seen mr and mrs chase this one off. Yet yesterday there were 2 misters swimming around happily together!!!
So I'm as

as you on that one but I'm just happy to see them and they get on fine with my domestic ducks.
As to the different colours - in some of my bird books they do show all sorts of variations of some of the more common breeds of ducks so it could be a hybrid. I can't think of another species with the same colour head as a male mallard. Unless it was a youngster who hadn't got all his adult colourings yet?