This is what I was told ..... I don't know if it is true or not...
A hen's eggs are siblings, like you are with your brothers and sisters. And, in the same manner that you produce children different than your siblings, so will the chicks from these eggs. Meaning, one should only incubate the darkest eggs, to continue the darkest genes are passed on. It made sense to me.
I have heard others say, "They have the genes to produce darker eggs," regarding the lighter eggs. I am not too convinced on that one. Simply because, in human terms, I know my siblings and I do not have the exact same genes or potential.
I have no experience for comparison of these theories; just passing on what I have been told. I currently have 6 of my very darkest eggs in the incubator, and plan to band these chicks and watch what they produce in the future. I am not incubating any light eggs, as I think it may be a waste of time.