I think an EE can consist of any mix of breeds as long as there are some blue egg layers in its ancestry. So no matter what you breed your EEs to, whether it's to each other or a purebred of some other breed, their descendents would still be considered EEs (even if they don't lay blue or green eggs).
Jumping to the topic about specific egg shade, from what I understand, the blue egg gene is dominant but it is carried on two genes. If a bird has both blue egg genes, the eggs will be blue. If it has one blue egg gene and one brown egg gene, the eggs will be more of a green. And if it has one blue egg gene and one white egg gene, the egg will be a lighter shade of blue.
So in short, people breed specific breeds with a known egg color together to get specific results. To get a darker olive shade for example, I think you would breed a bird with blue eggs, like an araucana, to a breed with dark brown eggs, like a marans. The blue hue still shows through but is diluted by the brown, like mixing paint.
If you breed your two EEs together, there's no good way to know what the results could be without just trying it out, because you won't know which dominant genes the rooster carries for egg color, nor the underlying genes present in either one. You might get a hatch with a wide range of colored eggs depending on the individual, from blue to brown. But since the vast majority of EEs lay greenish colored eggs, I'd say there's a very high chance your two would just produce the same thing.
Hopefully this wasn't too convoluted or confusing. I'm no expert but I hope my limited understanding could help a little. Good luck!