I don't think the Ringed Teal in question is a mutation. White in the heads of female ducks is not uncommon and may have something to do with age. It's pretty common in pochards, but has been documented in several other species.
Rosybill
Ring-necked Duck
Common Pochard
DT
The odds are against you when it comes to getting people to post mutation pics because as stated before, here in the U.S. we are limited on the number of mutations and most if not all have already been posted, meaning that most of the pictures of color mutations are going to have to come from...
I'll bite. Here's a few of my faves off the i-net.
Ruddy-headed Goose
Black Swan
Grass Whistling Duck
Northern Shoveler
Common Eider
Wood Duck (Australian)