I read somewhere that the pigment used for blue is Oocyanin which requires Lysine (an amino acid) as a component. Quinoa, Sweet Potatos and all animal proteins contain good amount of Lysine and lead to vibrant blue color. Now if all of them are getting the same diet but laying different shades of blue, then apart from genetic differences (that will require a lot of research), it might be the different rates of metabolism in different birds, so might try having separate pens with dark layers and light layers and feed the light layers with extra lysine containing foods and see if it makes a difference in 2-4 weeks. If it doesn't then maybe their glands are just not capable of secreting more than a certain amount of the pigment.There is some research that also indicates that nutrition affects the blue of eggs---and beta carotene can intensify blue. It could even relate a bit to the way the different breeds metabolize beta carotene - or how much grass/green forage the particular hens are getting -- Just thought I would add another complication to the equation.
:O)
One more factor to consider is whether the light colored layers are laying more eggs than the breed normally does. I believe the standard is 3.5 eggs a week on average. Now if they are consistently laying 4-5 eggs a week, that would also lighten the color because of either limitation of the gland or not enough raw materials in the diet.