As you know I read and much of the information I use for our conversation comes from PP mag. With that, it was the black orp that came first, by Cook to be a dual purpose breed. He used the Minorca roo and blk Langshan hen the offspring he crossed with the blk plymouth rock.
"While there are still a few utility strains of Buff in existence, the majority of birds around these days will prove to be relatively unspectacular layers. Unfortunatley , long gone are the days when a healthy Orp. hen would commonly produce 250+ eggs in a season". - quote
As for colors, " followed, white 1889, buff 1894, Jubilee 1897 , Spangled 1900, Cuckoo 1907, Blue 1908, and Red". (no date given). Keep in mind that we are talking the U.K. Things may be different here in the states. What is rare there may not be rare here. Our feeds are different for sure.
The only breed I have separate is my Dels. I have orps, but they are a mix of colors and with a few EE's and a couple of RIR's. IMO only keeping a separate flock of colors to themselves can we be sure just how well a layer a particular line is. I have tried to find "trap cages" for chickens here but have been unsuccessful. My brother used to have them for his pigeons.
I recently had a friend sell his flock of GC's due to low egg production. Keep in mind that management is definitely an issue. I just recently got one egg from my flock of Dels which I think may be due to having fed them some sweet potatoes. I can't be sure it was them or the other flocks that got the pan with the sweet potatoes in it. They have been picking up steadily since.
As for claims of "prolific" , that depends on whether your selling or buying IMO. It also requires accurate record keeping. After what happened with my purchase of SFH's and B. Marans? I trust no one in what they claim. I don't care if they or someone else call them expert breeders.