I did hatch my own Olive Eggers that ended up being a brown layer, but it's mom or dad was a Black Copper Marans so the egg is still a very dark brown. One of my other Olive Eggers is laying only barely a medium olive egg and it seems to have blue speckles on it which is weird.
I ordered Black Copper Marans eggs from a breeder back before the hatcheries started selling them because I had heard the rumor that Cuckoo Marans were not being bred by hatcheries for a dark egg color and were sometimes producing light brown eggs. My Black Copper Marans do lay a dark egg.
But yeah, you got a raw deal. Sorry. I would be disappointed, too. Fingers crossed for that one green egg layer that is supposed to be coming into lay in a few months.
The green egg layers have to be from purebred stock to guarantee a green egg layer. Some hatcheries/breeders are better about that than others. And sadly, I don't think any guarantee you a green egg layer, although I've had perfect results from both Cackle and Ideal hatcheries.
With Marans, there are a number of genes and modifiers (ingredients in a soup) that come together to make the very dark brown color. Unless the hatchery/breeder is really paying attention, those modifiers can drop out of the genetic pool and be lost and then the dark egg color is lost also. It can happen so easily before you even know it ... just losing one of your roosters can ruin it all if you are relying on just one rooster because the next rooster might not have that specific modifier for the dark egg genes to work together to make the dark egg.
The hatchery Black Coppers and Blue Coppers still seem to be making pretty dark eggs. Ask around. I'm a little out of the loop on the Marans these days.
So, while certain hens are supposed to be laying certain colors, they sometimes don't and I feel your pain and disappointment. You can start trying again right now by ordering some Black Copper Marans and some green eggs layers or you can wait till next year. But, still, sorry it didn't work out for you so far.