I have chicks from an ameraucana roo x black marans hen, how,when they are old enough,what breed of roo would i breed these back to keep the olive egg color?
To darken the egg you would breed to a dark egg layer ie Marans. If you have two Olive Eggers you can breed together you will get a "better" version of the olive with any luck. Some people do try to get them to breed true for egg color or for coloring on the birds. There are all kinds of ways to breed them.... go the direction of the color you are looking for.
Illia has a chart on post 3504 on my computer it is 6 pages back.
https://www.backyardchickens.com/t/131131/the-olive-egger-thread/3500