I have not given any thought of darkening the egg color & have not really been concerned about egg color. I will leave that to the breeders of the dark egg laying breeds: Marans, Welsummers, Barnevelders, Penedesencas.
I have noticed that most of my Buckeye eggs are a nice, sufficient brown color but as they lay further in their cycle (i.e. later in their laying cycle) (or the older the hen), the eggs lighten up to a light brown & yes, one here or there, an almost off-white. A new pullet lays a nice medium-dark colored brown egg.