Mike, if you want to understand egg shell color, you might read this, including all the links. In spite of the thread title it covers blue and green eggs too. A warning though. It can make your head spin.
The theory is actually pretty straight forward. It’s putting theory into practice that gets complicated. The base shell color is either blue or white. There is only one gene pair that determines that. Green or brown is just brown in addition to that base. You might think of it simplistically as:
Base blue + no brown = blue
Base blue + brown = green
Base white + no brown = white
Base white + brown = brown
There are a lot of different genes that affect the shade of brown. Some are dominant, some recessive, some partially dominant, and some only have an effect when another specific gene is present. You get all the different possible shades of brown because there are so many involved and they can go together in so many different combinations. Whether the base is blue or white is pretty basic but you rapidly lose control on shade of brown.
Egg shell color is like a lot of other traits with chickens. If the person that selects which chickens get to breed pays attention to egg shell color, they can control it. But if they don’t, egg shell color can go all over the place in a very few generations. Many hatcheries and many breeders don’t pay a lot of attention to egg shell color. I wish they would, especially for the chocolate and blue/green layers, but they often concentrate on other characteristics. If you are breeding for show and the judge doesn’t see the egg, why breed for egg shell color? Some breeders do, but some don’t. Each hatchery has its own people deciding which chickens get to breed. Some may pay attention to egg shell color but some obviously don’t.
I don’t know where you got your chickens, a breeder or a hatchery. I think that is what happened to your Marans. Someone did not pay attention to shell color and just lost it.
I’m not sure what shade “moss” is but to me that sounds pretty green, not blue. Your green egg layers should be laying blue eggs, but somewhere in their background they have received some brown egg genes. That may have been carelessness or it may have been deliberate if they wanted to introduce some other characteristic or even more likely eliminate another characteristic that they determined was a flaw. I don’t know what happened but your hens are not laying the color of egg shell that they should if they are true purebreds.
Did you breed these hens to a rooster from a brown egg breed? That would explain it, the rooster may not lay eggs but he contributes genetics to egg shell color as much and in some cases more than the hen.
The hen makes the blue and the brown color pigments from recycled dead red blood cells. Some people think the blue is made from bile but that’s not correct. The same chemical process that is used to make bile blue is used to make the egg shell blue but the raw material for the egg shells is recycled red blood cells. So, in theory what they eat should have no effect on egg shell color. I’m not totally sure that is true, though the following is speculation on my part. When a pullet first starts to lay or a hen starts to lay after a molt, those eggs are going to have the darkest brown you will see. But the longer they lay the lighter that brown becomes. I’ve had hens that lay a medium brown egg right after a molt lay an almost white egg just before their next molt. I think there is some chemical that the hen stores up when she is not laying but gradually runs out of over the laying season that she uses to make brown. She doesn’t run out of recycled red blood cells, that’s a steady supply. So it’s possible you could feed then something to keep the shells darker longer but I don’t know that is true or what that chemical might be.
Bottom line, your hens should not be laying eggs that color if they are true to their breed, but they are laying eggs that color.