"Is anyone able to tell me why different breeds of ducks can be identical colours? It's so odd to me that the colours can match so well and yet the body shape is different."
Well, it's sort of like you can have different breeds of dogs (ie labs, great danes, or greyhounds) and they can all be black.
And you're right about just about everything, except that ducks can fly. SOME ducks can fly, and a mallard is one of them! So are calls, East Indies, and Australian spotteds, along with, of course, any of the non-mallard derivatives like wood ducks, pin-tails, manderines, etc. Oh, and of course muscovies.
Your hen is beautiful. I miss mine, she was attacked by something (a raccoon I think) when she was sitting on her nest of about 15 eggs. I was devastated, especially since she was exceptionally beautiful (to me..... she was like yours. A light colored head. Most breeders breed for the fawn colored hood) I got some more this year and am hoping to have some babies next year. Do you have a drake?