Hi Meg
Swans can have 1-13 cygnets depending on species. We are not sure about the coloration you are describing as we have never seen a yellow green leg coloration unless the swans are covered in algae. The lighter coloration could be a leucistic coloration which is a variant, but quite normal. Could you please send us a photograph of the cygnets and leg coloration? Our email is Bolin.S @att.net
We will try to give you a better answer after seeing the photos. Thanks. The Regal Swan