It’s definitely possible, but it will take a few generations until you have what you want. If you want birds that will always lay olive eggs you will need to start by crossing say a Marans cock who hatched from a very dark brown egg and who’s sisters lay dark brown eggs to a blue egg laying hen. The F1 offspring will all lay olive eggs granted that your hen is homozygous for the blue egg gene. Now you would cross the F1s together to get F2s. However you will end up with 50% homozygous for the blue egg gene (green or olive egg layers), 25% heterozygous for the blue egg gene (green or olive egg layers), & 25% homozygous for the non blue egg gene (brown egg layers). Now you will take the F2s that lay olive eggs and you will need to test mate them to see if they carry 2 blue egg genes or just 1. To test this you will need to cross F2 cocks and hens to brown or white egg laying chickens. If an F2 Olive Egger is homozygous for the blue egg gene then all its offspring will lay blue, green, or olive eggs. If an F2 Olive Egger is heterozygous for the blue egg gene then half the offspring will lay blue, green, or olive eggs and the other 50 will lay white or brown eggs. Once you have ascertained which F2 Olive Eggers are homozygous for the blue egg gene you will cross them and your F3s will be guaranteed to lay green or olive eggs. I believe that it is hard to guarantee that the dark brown egg genes pass on to the offspring in F2 progeny. That’s why back crossing to Marans, Welsummers, or Barnevelders is done to improve olive egg color.