This is the flag attributed to the last Roman Emperor Constantine XI Palaiologos, defender of Constantinople.
The flag includes all the emblems of the Eastern Roman Empire:
a) the Cross with the inscription “εν τούτω νίκα” (with this you win) shown in the sky to the Emperor saint Constantine I the Great.
b) the double-headed eagle, emblem of the Empire
c) the “emblem” of Jesus Christ (the small one above the doudle-headed eagle). It is a combination of three letters: (1) I [for Iesus, Ιησούς, Jesus]; (2) XP [for Xpistos, Χριστός, Christ].
d) the four Bs standing for the phrase: King of Kings Reigning over those who Rule (Βασιλεύς Βασιλέων Βασιλεύων Βασιλευόντων)
The imperial purple and gold were the colours used.