This is normal across the board. Ducks with orange bills, blue bills and green bills all loose their pretty colors to darker browns and blacks as they lay eggs. I believe this is part hormonal and part nutritional and depends on bill color.
Entie May 14th, 2014 hasn't laid any eggs yet (2 months ago she was an egg):
Close to pristine.
Entie July 29th, 2015 of last year laying like crazy - (the red is watermelon):
Very dark and speckled.
Entie January 11th, 2016 before she started laying again:
Mostly clear still a little darker.
As a point of illustration on another color bill (they don't freckle as much as orange bills)
Here is Rhiannon on Aug 9, 2015 in full lay:
(almost totally black bill)
Here she is on December 3rd, 2015 she is on break (and got in a nice not-bleached-yet molt )
(you can see quite a bit of clean green in her bill.)
That is so cool! I would have never thought that their bills changed color that often and for those reasons! Ducks are awesome