Since my hobby is raising poultry, I did a little more research on this. I'm pretty sure they are referring to a duck like a Saxony, possibly a landrace or "farmer's breed" rather than a pure breed (believe it or not there are folks who take poultry showing just as seriously as those who show dogs, horses, etc.). Dave Holderread, a well-known US waterfowl expert, breeds the Saxony, and refers to this plumage pattern as "pastel". I've attached his web address for you, there is a photo gallery of the different breeds he raises and you can see for yourself what a Saxony drake and duck (canard saxe) look like. Other breeds, even crossbreeds, can have this pastel pattern too. In fact one of the breeds used to create the Saxony was the Rouen, which is a large meat breed originally developed in France that has the same plumage as the mallard (canard colvert). So I'd think it's highly likely that a farmer's breed or landrace of ducks with pastel plumage could have been found on small farmyards in France.