Tim, I believe the "coloured" Dorkings are supposed to be the same as the British "Dark" Dorkings. Dark Dorkings would be expected to breed true.
I have wondered about their genotype. They are almost certainly melanised e+ (Ml?), I've wondered about S with Mh?
I'm not sure whether US "coloured" Dorkins have the patterning. In the Darks I've wondered whether, possibly, some secondary pattern genes, maybe Pg, giving the half moon spangling on the breast & patterning on the wings of the females?
UK red Dorkings are not at all the same colour as what is known as red Dorking in US.