What breeds did you use? If you don't mind sharing?
My first two sets of Olive Eggers I used a cockerel from a project line of Legorn/Australorp/Ameraucana heritage (selected for blue egg gene) crossed over a Red Sex Link hen whose eggs weren't quite as dark as my current brown egg layer.
Certain strains of Red Sex Links lay fairly dark colored egg shells and maintain that dark coating despite prolific egg laying which is why I chose them.
My current set, due to hatch May 5th. This time the cockerel is a U of A Blue egg layer (Ameraucana --=/Leghorn hybrid project bird) The brown eggs are from an Amber Link, very similar to a Red Sex Link or Bovan brown/ISA brown. I don't think she skipped a day in the last 2 weeks of laying but her shell color is still nice and dark. We will see what her daughters produce!