I've seen the subject discussed. I'm not sure the answer is known. From my point of view I've never had the variation so have not had anything to investigate. Perhaps you could investigate.
The mottled Orp chicks on Feathersite look the same as mine.
Looking at your second lot I see they do look a bit blacker than mine; the black seems to go further down their sides.
I'd really like to know what causes the difference & the genetic significance, if any.
My mottled Japanese hatched out both ways and they still all turned out mottled
We're trying to ascertain why the down colour is different. Which e-allele are Mottled Japs usually, & which down colour is most usual? What colour are the legs of your mottled japs?
The lower chicks are what I usually see from my mottled Orps. They look usual for extended black chicks with mottling.
Those pale chicks are interesting. What was different in their father's background? What are the other chicks in with them?
I have also heard of some mottled chicks which...