Mine sometimes lay a green egg. Could be your Khaki. I know it isn't the Pekin but I don't have Mallards so I am not sure about those. My Runners also can lay green eggs.
I had no idea ducks laid anything but white eggs. And I didn't expect her to lay this year either, she is 5wks younger than the rest and I thought it was too late in the year to start.
olive green eggs... not as dark as olives, but definitely that color, that's what my mallards lay.
and the cayugas, yep! black as new asphalt, sometimes solid black, somtimes like they'd been rolled in asphalt hash - white with asphalt black about two thirds all over, patchy. they get lighter as the season goes on, but the first couple of weeks, black black.
my rouens lay very slightly pink eggs.
Khakis and Golden 300s, eggshell white, not paper white... just the creamy side of white.
runners, white or palest blue.
I love duck eggs!
festive on the outside, tastly on the inside.