Call duck egg color change

[inmyownzoo]

We have a new to us call pair. The hen has laid about 20 eggs so far this spring. She has a wild game crumble and access to oyster shell grit at all times. Earlier in the week her eggs changed from green to white- two white eggs and then yesterday a soft egg - basically no shell. I offer them grass clipping and dandelions. Is the color change a calcium issue? What else should I offer? Thank you