Jesus approaches the twelve and “blesses” them. We do not know what kind of blessing it is that they have received, but it has something to do with a purification, as evidenced in the next verse. After this blessing the twelve undergo a visual transformation. They become “white.” Just as fire is a visual image of the presence of the Spirit, so the light and whiteness used to describe heavenly beings is an indication of the presence of, and influence of, the spirit. When Jesus underwent his transfiguration we have similar language:
Matthew 17:2
2 And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light.
When the Spirit overcomes one, it can make its presence known in the form of light, an intense white light that appears to radiate from the person. That was the experience of Jesus on the Mount of Transfiguration, and it happens to the twelve on this occasion. Their great faith transfigures them into a more spiritual being, and that spirituality is visually manifest through light. Note the description of the angel who rolled back the stone on Jesus’ sepulcher:
Matthew 28:2-3
2 And, behold, there was a great earthquake: for the angel of the Lord descended from heaven, and came and rolled back the stone from the door, and sat upon it.
3 His countenance was like lightning, and his raiment white as snow:
The whiteness is not the color of anything other than the color of this intense light.