White clothing is the cheapest possible for itinerant preachers who give up everything as far as their vocation permits in order to follow Christ. White was also worn by the noblest of creatures, the angel who at the tomb announced the Resurrection of the Lord. he apostles, heralds of the Resurrection, also wore white garments according to the interpretation current in the Middle Ages. Later on, therefore, those who embraced the apostolic life would continue to wear the same kind of clothing. They lived a poor life and had no possessions; by their preaching they would announce the Resurrection to the entire world, all the world.
-Josef Siegwart, "Origine et symbolisme de l'habit blanc des Dominicaines"