They can change from green to blue because the brown spray paint has run out or temporarily stopped..actually the shell will always be blue.and the Ameraucana cross lays large green eggs...
... until today. Today she laid a medium sized, light blue egg. I know the light blue egg has to be from her and not one of the other hens.
So I’m convinced their egg color can change.
But a chickens eggs cannot change from blue/green to brown...
.....or from brown to blue/green.