I'm not sure, but if the parents have the lavender gene (recessive) you might be able to get some lavender ducklings.
In 'Storey's Guide to Raising Ducks' it doesn't mention lavender genes in Mallard-derived ducks, but in Muscovy ducks, lavender is the result of a duckling getting the recessive lavender gene from each parent. If a duck has only one lavender gene, it won't look lavender, but if it mates with another duck with one lavender gene, there is a one-in-four chance of a lavender duckling.
I would imagine that the gene is probably the same in Anconas, so if you have the good luck of having some with the recessive lavender gene, you may have lavender ducklings.