Great post, what do you mean by Historically significant varieties?
There are varieties within these multi-varietied breeds that are the reason for which the breeds hit the map in the first place. There are others varieties that always enjoyed a level of popularity in a way riding the coat tails of the original varieties. Then, there are other varieties that never really were anything but a project.
Take Dorkings. Whites are, within dependible recorded history, the original variety, and were the variety that made the reputation of Dorking meat. Eventually, SG Dorkings rose to the fore in the US. Red Dorkings have been a constant presence in the side lines. Colored Dorkings have probably sung their last song. Cuckoo Dorkings have never had any real historical relevance in comparison to the other varieties.
A similar list could be built for ther multi-varietied breeds, and certain breeds "own" certain colors.
Buff: Cochins, Orpingtons.
Barred 1: Rocks
Barred 2: Campines
Cuckoo: Dominiques
Partridge: Cochins, Chanteclers
Ermine or Columbian: Brahmas, maybe Wyandottes
Blue: Andalusians
White: Dorkings, Rocks, Wyandottes, Leghorns, Chanteclers
Red: RI and NH
BBR et al: OEG, Leghorns, Dorkings
Black: J Giants, Australorps, Orpingtons, La Fleche, Crevecoeur, Minorca, Spanish, OEG
Silver Duckwing: Dorkings, OEG
Speckled: Sussex
Mottled: Houdan, Java, Ancona
Spangled 1: Hamburg
Spangled 2: OEG, Aseel
Laced: Wyandotte
Double partridge: Cornish, Bernevelder
Silver Penciled: Brahmas
Take Buff Rocks. They're awesome. My folks had some growing up. They certainly had their hay day. But they're still one of those varieties riding the coat tails of the principal varieties: Barred and White. If Buff Rocks disappear, it's a sign of the times, but it's not the end of the line. If Buff Orpingtons disappear, there's a real blow. In the same way, Barred Rocks disappearing would be poultry tragedy.
Blue Rocks disappeared. They never had a place to begin with.