Hi Morbius18 .
I think the your chick GCM with a Brown face when hatched and more orange at Adult because he carry the combination of Co Columbian restrictor and the Dark Brown Db.
i suspect the atavistic return of the eb bird carying the Columbian where mixt to the original e+ stock.
the Co is responsible for turning the Wheaten yellow hatch down to buff with the addition of the Db we ll get the Black tail buff Marans.
Co is not dominant to the ER so does not change the chick hatch down by it self does need other modifier gene like the Db or Pg ( patterning genes ).
Only the ER looking chicks ,lighter cockerls ( blue grey with a larger dot ) at hatch and a darker pullets ( black with a little dot ) are the proper phenotype for the GCM
So any modifying gene should be breed out to purify the ER hatch down of the chicks.
the Mh does not affect the chicks hatch down but will react with the other Autossomal red genes in the adult pattern .like dark hackle and lancet of the copper Coq or the Wheaten .
Hope that help my friend
Cheers Chooks man