The brown on brown eggs is "painted" on as the egg passes through the oviduct. Darker eggs have a thicker layer of "paint". To achieve this the egg passes through the oviduct slower. Someone else may be able to provide a better explanation, but that is my take.
I am not too far from you - Birmingham
I have two 3 day old pullets and I have to say that they are lively! I am looking forward to watching them grow.