Opinionated people or people who don't like me, go ahead and rage about it, but - I'd use Ameraucanas.
Why? Their bodies actually compare better than Orpingtons, and if you've noticed, when you get single comb flukes from Olive Eggers or when you keep breeding OE's down the line, you get some very nice dark brown eggs.
Now of course, there are indeed other traits to deal with, but to me it's a closer road than Orpingtons. And the egg color is a big thing here. Along with of course the main body shape. Yes, most Lavender Orpingtons are seriously far from either American or UK standard and would do okay for Marans, but, I see more Ameraucanas fitting than project Lav Orpingtons. My two cents. Either way it is indeed a long project that requires tough decisions and hard culling.
Personally, really, I just wouldn't do it.
I'm tired of seeing Lavender hit every breed out there. I'm actually working on a new color introduction to Marans, it will take a long time yes, but it isn't Lavender.
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I think you oughtta show me some pics.
Wheaten is easier to work with than Copper when it comes to Blues and breeding, however, there aren't too many seriously great Wheaties out there, and a lot of them have not the best egg color. But that aside, if the birds are great, just remember to cull hard. Wheaten is recessive to Blue, and will look like a Birchen or a Lemon-Blue when crossed with it, so down the road in the project, cull out ANY suspected leakage. And if you have a Wheaten pop up, cull both parents.