I think if you want to be sure you have olive eggers you either have to start with "pure" blue egg layers or you have to grow them out until they lay and you can see the color for yourself.I think it depends on your market. If you are selling to the hobby farmer who wants the olive color in the egg basket, they would not be happy with you even when you tell them the bird may lay brown. If selling to someone who is interested in genetics and breeding, they would understand the numbers involved and what they "might" get from those birds.
Because I'm nearer to urban areas (25-50 miles from my remote location) where laws allow no roosters, I chose single comb olive eggers and only sell the F1 (first generation) crosses. They "should" all lay olive eggs. I also have some that are sexlinked, so I can confidently sell baby chicks as pullets from those birds. These are Black Copper Marans over cream crested legbar. The others, also having single combs, show themselves as pullets anywhere from 3-6 weeks. These are Isbar over Marans.
Question: Do you cull the single comb chicks of your F1 crosses just to be sure they are OEs?