Very dark yolks

[maflood]

Do you mean closer to black yolks?

Yes, here it is

 

[Smokerbill]

Have you seen this thread? It mentions that a weed called shepherds purse, raw acorns and/ or cottonseed meal can cause green looking yolks.
https://www.backyardchickens.com/threads/green-egg-yolks.1111515/

More about green duck egg yolks...
https://www.adelaidechickensittingservice.com/blog/what-causes-brown-or-green-duck-egg-yolks