I have personally butchered many, many chickens over the years for the table or freezer. All I can say is that it is not uncommon to have birds with yellow or willow shanks that also have a pretty white skin elsewhere. Usually you can see a little evidence of the xanthophyll (yellow pigment) if you look hard enough for it. As you probably know, the yellow pigment gradually disappears in a laying hen, because egg production removes it. I guess it probably goes into the yolk? That's why poultry judges are lenient in "yellow" legged hens and pullets that have been laying if they have pale or even whitish legs.
I think the best way to eliminate the yellow epidermis gene in Ameraucanas, if you have the space for it, is to keep a few of those willow legged birds for use in test mating with your potential breeders. Of course that means raising up a good number of chicks to the age where shank color can be determined with certainty, and none of them will be keepers, because even the slate legged ones will be carriers. The other way would be to keep track of which male a group of chicks is out of. If ANY of them develop willow shanks, you know that rooster is a carrier, along with whichever hen the chick is out of. When you breed a slate shanked carrier to another slate shanked carrier, the odds are that 25% will have will willow legs, 25% will have slate legs and NOT be carriers, and 50% will have slate legs but be carriers. So the odds are not that good. We went through this in developing the bantams 40 - 45 years ago, as some of the outcrosses involved yellow or willow shanked birds that had the proper color pattern. For example, I once used a Silver Leghorn bantam in the attempt to produce Silver Ameraucana bantams. And the bantams Jerry Segler received from Ralph Brazelton had willow or yellow shanks.
Once you have identified a carrier rooster, you could use him to individually test mate females to see which of them are also carriers. If a female turns out to be "clean" (a non-carrier), all the chicks from that mating will have slate legs. But about half of those will be carriers, because they will have inherited the yellow epidermis gene from the rooster. The other half will not be carriers. In wheatens and blue wheatens, you can often tell which is which by examining the color of the shanks at about 3 to 5 weeks of age. The carriers will often show a bit of willow at that age, even though later in life they end up having slate legs. Just compare the chicks from the same matings at the same age. I think you will be able to see the difference.
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