Dark spots on duckling feet

[Aussie dog]

My eight runner ducks are three weeks old and a couple of them are developing dark spots on their feet. I'm afraid that it might be bumble foot. The ones that have it are either chocolate or blue and some of them have dark spots in their feathers as well so I'm hoping that it's just new pigmentation developing as they grow.
Most of it is on the tops of their feet but there is a little on the bottoms of their feet too. They're all acting normal and don't seem to be in pain but if it is a problem I want to treat it before it does start to hurt them.
Here is a picture of the worst of it

 

[EverythingDucks]

Here's what I've read on this

"-melanin skin pigment, which is not harmful in any way. ... Less common causes of duck bill and leg discoloration include injury, infection, dietary deficiency, and possibly, fungus. "

Hope this helps!