What should Rouen and Cayuga ducklings look like?

[Quacking Pigeon]

So I was looking through Gumtree today and I saw these 3 ducklings for sale, 2 were Rouens and one was a Cayuga. Thing is the Rouen ducklings look a bit too yellow to be Rouens and the Cayuga I think looks like it should. I'll see if I can find any photos of ducklings which look similar to the Rouen ducklings on the internet.