Every Bushi has a different colour soul, and when a Bushi touches a Kitetsuto the materials in the sword shine and reveal the colour of the soul. In Kojiros case he shone blue, which was revealed to be a common colour since 2 out of 5 bushi have blue souls. In total there are 5 different colours red, white, green, yellow and blue. Of the five red and white are the most rear. Most Bushi are one of these colours but there is one known exception, that being Musashi who has a black soul. Those with black soules are known as Imibito, the abhorred ones. Those with black souls will never be acknowledged by a Kitetsuto.