You and me both. On roos that turn lighter at maturity, I understand that has to do with testosterone... but not *how* to breed that out nor the exact details(genes) necessary for that.. know it can be done because there are tons of silkies, cemanis with perfectly dark skinned mature roos.
Used to think those roos were not pure Fm, yet had a few roos out of Fm bred with normal (so could not be pure Fm) that stayed dark and didn't turn red or roos that could have been FmFm by being pretty dark skinned and out of Fm parents.. yet they threw normal skin chicks. I'm at a loss about this.
don't think Id is involved in those kind of cases, they usually mess up the Fm on skin right from hatch, either very patchy skin like a Holstein cow or the skin is so light it's flesh colored and you know it has Fm only because it has things like grayish skin on face, inside of their cloacas are gray/black and/or as they grow you can see their black bones showing thru(usually the skull, neck bones and parts of the esophagus).
could test them for Id by breeding with non-Fm and non-black chickens with green or slate legs, because if they produce any yellow or white legged birds, then Id is definitely present.
as for Fm pair throwing normal skinned chicks I simply mark both of them as Fmfm Fmfm.
Congrats on success as for the colored eggs! It's so cool you have something added to 'em besides just being NN Fm.