I had a few older rir awhile back.  Depending on where on the farm they ate is how dark the eggs were. 
When they roamed by the manure pile, they were darker than if they ravaged my garden and compost.  While everything was covered in snow, the eggs went light to light/medium brown with spots.
    I only had about 10 chickens, so I am pretty sure they didn't take turns laying different colors over the months.  I know this doesn't help much in your quest to figure things out. 
  Maybe this time you just have to sit back and enjoy the fact that you can DYE lighter eggs but not the darker ones...Easter is coming!