I've had one of my older EEs miss putting on the brown coating before, all of a sudden. One day, she laid a yellowish green, the next egg was greenish blue, and she continued to lay eggs that colour for a while. I'd say it was a few weeks, but I didn't keep track of it, and this was a couple years ago. I know they were hers because she had a peculiar tendency of putting a huge deposit of calcium lumps on top of the egg. Pretty unmistakable.
Her "blue" eggs weren't speckled, mind you. And after moult, they were back to being green.