I hadn't thought to Google but it is hard to trawl through all the blurb containing Dr Seuss, those preserved chinese eggs and hard boiled eggs with a green tinge!
I've googled Shepherds Purse, and whilst I may have seen it here, it certainly isn't one of my common weeds that I recognise straight away. 
 
The ducks laid 3 eggs this morning, so I'll be cracking them all to see. Yesterday I had one green one and one that was yellow-but-not-quite-right.
Just found that the plant Pennycress can cause green yolks too. Whilst I don't think I recognise that either, what I do have a lot of on the beach area by the pond, where the ducks spend a lot of time, is hairy bittercress which has very similar flowers and sounds like it is from the same family. I wonder if it has the same effect on the eggs?