I answered a similar question only a short while ago.

A member of the Byzantine elite in the 13th, 14th and 15th century would have dressed very differently than a Roman senator in the Republican, Imperial or Late Antique age. One of the most obvious differences would probably be the disappearance of the toga, the signature piece of clothing of the antique Roman citizen. Once it had been so strongly tied to Roman identity that the Romans were sometimes even called the gens togata. Late Antique portrait statues of senators and emperors still show them proudly wrapped into their toga. Only in the 7th century did this change markedly when portraits of people in toga more or less vanished. This is probably connected to the severe restructuring which Roman society underwent at this time. As a result of the invasions by Persians, Arabs, Slavs, Avars and others the Empire had lost massive amounts of territory and with it the huge estates of the senatorial elite which had been dispersed throughout the provinces. The state was now dominated by men of much humbler origins like those that had climbed the ranks in the military. It seems that when the old senatorial elite vanished so did the piece of clothing most strongly connected to them.

Another garment with high symbolic meaning survived this period of change. The chlamys was a long cloak which was fastened with a brooch above the right shoulder and normally signified its wearer as a member of the Late Roman bureaucracy or military. Administrators, senators, generals and even emperors were often depicted in the chlamys. However in Late Byzantine times it is only very rarely seen in portraits from that time and seems to have largely fallen out of style in the 12th century.

Late Byzantine dress, at least at an elite level, was dominated by long tunics and kaftans that reached all the way to the ground and which were belted at the waist. They were also much more colorful and intricately decorated than what we know from ancient portraits. In this regard Byzantium was very much a part of the wider Eastern Mediterranean and Middle East. People in the Islamic realms as well as in the christian principalities of the Caucasus were wearing very similar pieces of clothing. This is also true for headgear. Turbans seem to have been a familiar sight in Late Medieval Byzantium. All in all clothing was probably much more influenced by Byzantium’s neighbors to the east and south than by those in the west. However that doesn’t mean that there was no western influence at all. For example according to Nikephoros Gregoras people at the court of emperor Andronikos III (1328-1341) were wearing clothing influenced by styles from Italy, Serbia, Bulgaria and Syria. Especially the flamboyant head pieces that Late Byzantine nobles had taken a liking to were probably incorporating influences from several different directions.

So all in all Late Byzantine clothing was much more similar to that of other Late Medieval societies than that of ancient Rome.