That is very interesting. I have a couple of young Welsummer pullets, too. I will have to wait to see how their eggs compare to the BC'smy experience has been breeding different hens to an EE of unknown parentage. In the group of hens, two were pure Welsummer, two were Wellie/leghorn crosses and one was an Australorp/EE cross. Of the dozen or so pullets laying from this breeding about a fourth lay some shade of brown; from pale like a buff orpington to a dark brown like a Welsummer. I would guess in your case you will get about three fourths or so of the pullets laying some shade of green and the rest some shade of brown if you breed him to a black copper or a welsummer.
In the case of your green layer, do you have the mother or sisters of the roos in question? I would breed to which ever carries the darkest egg genetics if I wanted olivers. Again realizing some will lay brown.
It sounds like it's important for me to carry on the genes of the EE'r that lays the nice green egg.
I do have the mother of the mixed roo.
Thank you.