Making green egg layers is easy, and brown egg laying breed crossed to a (purebred) blue (or green) egg layer (Ameraucana or Cream Legbar, for ex). It doesn't matter which is the roo, egg colors are not sexlinked (that anyone has noticed, anyway). But if the blue/green parent has only one copy of the blue gene, then half of the progeny will lay green eggs and half brown.
Olive eggs are just a special case of green where the brown egg layer is a "dark egg" breed, Welsummer, Marans, Pescadena, etc.
Because both brown and blue are dominant, this first cross is easy and can be guaranteed to produce 100% green eggs. Turning that into a true-breeding strain is a lot harder, if you cross 2 F1 OE's, you will get 25% that will lay brown, not green eggs. And the brown may lighten in many of the progeny as well, as the genes re-mix in each generation. It would take a lot of work to get a consistent egg color in a true breeding strain of OE's. I don't know if there is enough demand to warrant that, I feel like OE's are a bit of a fad, personnally, I think a good blue or dark brown egg is much more attractive than an olive-drab or khaki egg, so I'm breeding toward the bluest and darkest brown eggs, knowing I can create F1 OE's on demand.
Oops, seems that I was confused about breed sexlinks and green egg layers.
I thought all EEs from hatcheries are F1 hybrid birds. Do they breed EE to EE? Then how can anyone know which 25% lay brown eggs?