Do you want to test mate your fibromelanotic birds to see which ones are homozygous for the trait? If so, the reason you would want to use slate or willow legged birds is because they have the wild type leg color that won't interfere with your test matings. Birds with light legs (white or yellow) have a sex linked dominant gene called inhibitor of dermal melanin. This gene blocks dark pigment in the skin and legs to a large extent, especially in young birds. This will make it look like your potentially homozygous fibromelanotic bird isn't passing the gene onto all of his offspring even though he is.
Additional comment:
Because the inhibitor of dermal melanin is sex-linked, a rooster can be tested using light-legged hens. The chicks will be sexlinks, with daughters having dark legs and sons having light legs. So ignore the sons, and look only at the daughters to see if they have fibromelanism or just normal slate/willow shanks. (This means you have to hatch about twice as many chicks for a good test, because half of them don't tell you anything. And the "dark legged" daughters may appear to have light legs when they hatch, but should darken up nicely within the first few weeks.)
For testing hens, you do need a rooster with dark legs, because a light-legged rooster will produce light-legged chicks of both genders.
Now, I see that you suggest using Ameraucana for testing but I do not have that breed. What I have are: Silverudd's Blue, Welsummer, Black Copper Marans, Cuckoo Maran, Beilefelder, Anconda, Silver Wyandotte, and multiple olive eggers.
I have tried using the Kippenjungle.nl website to identify a suitable pairing but was unsuccessful.
I know you did say to use Slate or Willow shanked birds but could you tell me if any of these will work?
Many Easter Eggers and Olive Eggers do have slate or willow legs, so you might start by looking at the legs of your Easter Eggers to see if any of them would work.
Also look at the legs of each Welsummer, Bielefelder, and Wyandotte. They are supposed to have light legs, but "wrong" leg color sometimes happens. (I once had a Bielefelder chick with willow legs.)
For the Marans (both colors) and Ancona, the legs will probably show quite a bit of black color because of the genes that cause their dark feathering, so you probably can't tell whether much else about their skin color genes. The Silverudd's Blue might have that problem too.
I'm curious, do you remember why Nicalandia recommended slate shanks to test? I have no hope of finding that post however far back it was, this thread is huge
I don't think it's in this long thread at all.
https://www.backyardchickens.com/threads/ayam-cemani-test-breeding.1403389/#post-23058387
^Here is a thread with quite a bit of information from nicalandia about test mating, with reasons and explanations. Start at about post #8
https://www.backyardchickens.com/th...e-sikie-and-ayam-cemani.864273/#post-13052587
^Here is a thread with a post from nicalandia that mentions Fm1 and Fm2 as two different genes causing fibromelanosis. It doesn't give many details, but seems very clear that there are two such genes.
