Almost 5 week old rouens getting iridescent green feathers

This weekend, when the sun finally came out, I noticed that one of my 4 1/2 week old Rouens has iridescent green feathers on his back, just above his tail. Then I noticed that his head also has iridescent green. I couldn't see it until the sun was shining on him. He also sorta has a muddier looking bill. Other than that, he looks just like the other 4 duckies--feathering in brown/black speckled.

I thought the color didn't come in until later, so I was surprised to see the green. Girls don't do that, right? That means he's a boy, right?

I'm getting miscellaneous quacking, but I can't ever tell who it is.

Not that it really matters, I just wondered.