Alright, I've gotten confirmation that Drako is chocolate mallard aka Nutmeg. Might be a blue gene in there too.
So here's my working theory. You have what appears to be a chocolate bibbed drake, and Drako is a chocolate/lilac mallard drake. That can't be possible unless their mother was chocolate, right? So, there's a chocolate female in your flock. And it must be one of the ones that could have mothered the chocolate bibbed duckling. Which brings me to...
Thea isn't pastel. She's lilac mallard. Aka blue with chocolate.
She fathered Drako, and gave him a chocolate gene. Then, if he was bred back to her, they could produce both female AND male chocolate ducklings.
But if she's chocolate, then it means Duck must carry chocolate. Which means he's been passing chocolate to some of his offspring. That would result in some females that are chocolate, which is hard to see on a mallard base, and some males carrying chocolate. So Ralphie could also be carrying chocolate. When then bred to Thea, that would produce some chocolate drakes - which could make Drako lilac mallard, which is why his head is brown. He got chocolate from both Ralpie and Thea.
Meanwhile, Ralphie fathered the blue bibbed drake. He passed him his chocolate gene. Blue bibbed then mates with Thea who is really lilac mallard and boom, chocolate bibbed drake.
Of course, you can only get just chocolate and not lilac or lavender if Thea has only one copy of the blue gene, not two. So my theory is she never was pastel. The chocolate gene dilutes black just like blue, making her appear light enough that we thought pastel even though her mother wasn't blue at all.
A little bit of test breeding can prove this. If you separate Thea and breed her only to the chocolate bibbed drake, if you get chocolate ducklings, we know this is true.