Welsh Harlequins Male or Female?

[Quackers912]

We have four Welsh Harlequins just over 6 weeks old. We thought they were all males based on coloration, but I'm pretty sure I heard one of them quack. They don't make much noise except for chirping so it's hard to tell based on voice. Do females sometimes start out dark like this or are they likely all male? Any help is appreciated!

 

[lomine]

I've had a couple females that had dark heads. Not green like a drake, but a darker gray than their necks.