The Scripture itself tells you what the "Great City" is -- it's the city "allegorically called 'Sodom' and 'Egypt,' wherein their Lord was crucified".
That ain't Rome, pal. There's only one possible candidate.
The language of Rev 18 echoes that of Jeremiah 25. Compare them! Jeremiah 25 is clearly not talking about Rome.
Scarlet [is] the color of the Beast and the Harlot, and also of the Dragon [Rev. 12:3], [and] is the color of the Papacy."
The Pope's coat of arms have no scarlet in them; their colors are silver and gold.
The Cardinals hats and robes are scarlet.
The Jewish High Priest wore both scarlet and purple. Look it up.