Hi John,
I don't have any experience with Muscovy ducks I'm afraid but other breeds usually have their adult plumage after a year.
It's amazing what genetics can do and what can be hiding. I bought a pair of very nice Abacot Ranger ducks and this year, the offspring have a few interesting things popping up = a couple have the wrong bill colour and all youngsters are heavily marked. After several discussions with Chris Ashton, the most probable thing that has happened is that they were crossed with a Welsh Harlequin or a Whalesbury hybrid that was popular in the 70's but who knows... all I can do is keep selecting the best birds. The parents are pretty good specimens and who knows what will pop out next!
Tim