Very late reply but how is your breeding program progressing?
I've worked with these a long time now, and the problems with Pyncheons and the SOP go beyond just shank color.
You've probably noticed that.
They are very hard to find.
If you are breeding them and intend to just address shank color to start, there is no shame in using yellow-legged birds to initially outcross.
The white shank alleles the modern Pyncheons display is incorrect, and unfortunately a dominant gene.
To address the problem, use a first-gen cross with a yellow-legged bantam.
Also, I would highly recommend you use a red-ginger male that is yellow-shanked for your initial cross to correct ground-color on the Mille Fleur pattern.
It is difficult to find the red-gingers, but they are out there.