Sorry, I know absolutely nothing about Cream Legbars, but a mixed breed is a mixed breed. Not all mixed breeds are Easter Eggers and not all selectively bred birds are Araucanas or Ameraucanas. It is really not that hard to understand. An established breed is something that is approved by the APA or ABA for the most part or in the process or working towards approval. There are several breeds, varieties, and breeds/varieties working towards standardization. Likewise, there are numerous mixed breeds, most with no names at all. The term "Easter Egger" is one of the few terms for a non-standard/mixed breed chicken, most are not given a name even. The EE are simply birds that have some Araucana or Ameraucana blood (or shared the common ancestors of those now established breeds). That's all. It is really not complicated, even though it seems so at first. LOL. You will understand after awhile.
To be standardized, a breed has to have consistency. Standardized breeds are typically recognizable for a certain type (in basic terms, type is for the most part the shape of the body and physical characteristics) and color(s). Mixed breeds are not (recognizing of course, standardized varieties certainly can start as mixed breeds). Selective breeding just involves choosing birds that fit within the approved type and color (which are specified in writing in the Standard of Perfection or SOP as it is typically known and is much more involved than just egg color). Breeds that are working towards approval are also selectively bred. There you have it. Araucanas and Ameraucanas have gone through that stringent process of becoming approved. Easter Eggers never have simply because they vary too much in type and color and people still like to mix in other breeds, in part to create different eggs colors. Sometimes people also have a tendency to think that the standardized breeds of poultry are the older breeds that have been around longer than the non-standardized breeds/varieties. This is often the case, but not always. For example, Dutch Hookbills are one of the most ancient breeds of ducks, easily having been bred for over 200 years and they are *not* a standardized breed (and they also breed quite true to type and color).