So I guess if an Ameraucana doesn't breed true 50% of the time it becomes an Easter Egger. And as you said, only certain colors breed true. So you CAN cross Ameraucanas and get an Easter Egger. To me, that means they aren't really a breed, and makes me wonder how the breed continues on...but we humans are kind of silly sometimes!
You can't cross different color varieties and still call them Ameraucana. This is true for all chicken breeds, not just Ameraucana. Chicken breeds are not based on pedigree and lineage, but on whether a bird with a certain set of characteristics, when bred to another bird with the same characteristics will produce more birds with those characteristics or not.
For example, if you cross a wheaten Ameraucana with a solid black or blue Ameraucana, you will produce birds that are not wheaten or solid black/blue; and furthermore, those chicks will be complete wild cards when old enough to be bred.