The clothing in da Vinci's Last Supper was much more Roman than what Jesus and his disciples would have worn. During Jesus' lifetime, probably AD 5 or 6 to AD 29 or 30, Judaea was still very influenced by Hellenism. Greek is the language of the New Testament, and Greek culture is reflected throughout the stories of Jesus' life. The word for garment used in the New testament is Chiton. A chiton in classical Greece was a short tunic worn under a cloak, however New Testament scholars believe that it was a long garment at the time. Along with this, there likely would have been a cloak worn over the chiton, not resembling a Roman toga as da Vinci shows. The cloak would perhaps have looked something like this: http://www.ancienthistorylists.com/wp-content/uploads/2014/08/Chlamys-ancient-greece-clothes.png it is important to realise that da Vinci's artwork portraying characters from the Bible will be heavily based on their iconography to make them easily recognizeable to an observer at the time. Portraying Jesus and the disciples in Roman clothing goes back as far as the second century, at the dawn of Christian art. Da Vinci would have been doing this both because of the sacred tradition, as well as because of what people believed the ancient world looked like. They used what they had to base their pictures on, but also took liberties for creative license. Roman artifacts were common, and portraying Jesus in a Roman style would have been considered historically accurate, and also politically correct for a Roman catholic church.