I'm thinking Cayuga and buff or khaki. There was a buff mother duck at our lake who had a few ducklings that were that lighter yellow color and they turned out to be full buff or buff hybrids. Both could be mixes. The duck in my avatar was originally dark like that, but now she looks more like a mallard. The avatar is her awkward teenage feathering.
Now that I think about it, I knew another mother buff who had a duckling that lighter color and he/she was a mix, too.