As others have said, egg color will lighten as the hens lay and use their stored pigments up. Selecting for the brightest color eggs in hens that have been laying for a while can cause you to select for the poorest layers, as, since they lay fewer eggs, their eggs maintain a more saturated color. This has unfortunately happed in some cases with Marans, leading to lines with hens that lay fewer eggs. Egg color is best determined in birds just as they start laying, and should not be expected to stay as dark as time goes on.