Is this just a chicken semantic thing?
To a certain extent I think it is semantics. First there were wild chickens. Mankind domesticated them and learned that with selective breeding they could improve egg-laying, size for meat, or keep certain decorative things like a crest. So different "types" of chickens were developed. Then some people wanted to have a contest, who can breed the best chicken. To let the judge know what the rules were they wrote a description, what we call the Standard or Perfection (SOP). That's when breeds were born. Eventually countries developed "associations" that standardized breeds. Interestingly different countries can have different descriptions of what constitutes a breed. The UK Auracana can look a lot different from the US Auracana for example. I don't think the UK even officially recognizes the Ameraucana as a breed. Different countries recognize different colors/patterns too. Breeds are totally a manmade thing, nothing really natural about them.
Some recognized breeds have only one approved color/pattern. Some recognize several different color/patterns. There are several projects going on to either get certain breeds recognized in the US that are not currently recognized (Cream Legbar for one) or to get a new color/pattern recognized for a recognized breed (Black Mottled Ameraucana) is one I'm aware of.
I'm not going to copy an SOP, that would probably be a copyright violation. The way the write-up for different breeds is organized can vary in the SOP but I'll try to show a standard one.
First they give a pretty precise description of how certain words are described. What do they mean by cape, beard, or barring for example. That's pretty standard in a legal document. Basically give a definition of key words they will use. This is how the American Association defines Breed.
"Breed: A race of fowls, the members of which maintain distinctive shape characteristics that they possess in common. Breed is a broader term than variety. Breed includes varieties, as, for example, the Barred, White and Buff varieties of the Plymouth Rock breed.
I could not find a definition of "purebred" in the SOP. I've heard that chickens need to breed true for five generations before they are considered purebred but I think that is just a rule of thumb, not a hard and fast requirement.
Then they describe the requirements of a specific breed. They give STANDARD WEIGHTS of Cock, Hen, Cockerel, and Pullet.
Then they give the SHAPE OF MALE with pretty precise descriptions of the various parts. Head: Beak: Eyes: Comb: Wattles and Ear Lobes: Neck: Wings: Back: Tail: Breast: Legs and Toes:
This is how they describe this particular comb, for example.
Comb: Single, rather large, set firmly on head, perfectly straight and upright; five well-defined points, those at front and rear smaller than middle ones; fine in texture; blades free from serrations.
Then they give an equally precise description of the SHAPE OF FEMALE.
Then they go into specific requirements for each approved color. For a specific color/patrtern they describe what are automatic disqualifications. Then they go through the COLOR OF MALE. Beak: Eyes: Comb, Face, Wattles and Ear-Lobes: Shanks and Toes: and Plumage. I'll give an example of the plumage description for a buff male of this breed to show how precise they are.
Plumage: Surface throughout an even shade of rich, golden buff, free from shafting or mealy appearance; the head, neck, hackle, back, wing- bows and saddle richly glossed. Under-color, a lighter shade of buff, free from foreign color. Different shades of buff in two or more sections, is a serious defect. A harmonious blending of buff in all sections is most desirable.
Next they go through a detailed description of the COLOR OF FEMALE.
Then they go through this process to define each different color/pattern in all the approved color/patterns.
If you have waded through all this congratulations. It can be a tough slog. I don't think this is a total hijack of the thread because to discuss whether a single breed is better than a mixed flock you need to know what a "breed" is. I looked at different dictionary definitions of "purebred" and "purebreed". Different dictionaries gave different definitions. Some include variety, some just say breed. So use whatever definition you wish.
But I'll go back to what my original point was supposed to be. If a reason you want a purebred flock is to sell hatching eggs or chicks, the value of those eggs or chicks will diminish if you mix different colors/patterns.