The American Poultry Association has a list of what color varieties they accept for each breed:
https://amerpoultryassn.com/accepted-breeds-varieties/
Some breeds also have a few other colors in development, that are not on that list. But general multi-colored mixes that do not match any color variety of any breed, are not going to be purebreds of any breed. Other traits matter too, but color is usually one of the easiest to see in partly-grown chicks that do not yet have their mature size and body shape.
For these chicks, they do have a number of traits in common with Ameraucanas (comb type, foot color, no feathers on the feet, no crest on the head, and so forth), so if you don't know what colors the breed can come in, it was definitely a resonable guess.
But Easter Eggers being called "Ameraucana" is a sore subject with many people (including me) because so many hatcheries are selling chickens with wrong or confusing labels (like "Ameraucana" or "Americana" for mixed-breed chickens that lay colored eggs but are definitely not actual Ameraucanas.) There is nothing wrong with Easter Eggers, and I actually like some of them better than pure Ameraucanas, but I have major problems with deceptive marketing.
That design in neck feathers is quite common in mixed-breed chickens as well as in some pure breeds. So I'm not surprised to see it in chicks that are probably Easter Eggers, and also to hear that your Wyandotte-mix has it too.
It can definitely be hard to tell what is (or is not) in a mixed-breed chicken, because the genes rearrange to give so many different appearances.