Mormon does not specify how Jesus “blesses” the twelve, but it has something to do with purification (v. 26). Then the twelve undergo a transformation, becoming “white.” Just as fire is a visual symbol of the Spirit’s presence, so the light and whiteness used to describe heavenly beings indicates the presence and influence of the Spirit. Jesus’s transfiguration is similarly described: “His face did shine as the sun, and his raiment was white as the light” (Matt. 17:2). The angel who rolled back the stone from Jesus’s sepulchre had a “countenance… like lightning, and his raiment white as snow” (Matt. 28:2–3).
This brilliant whiteness is not a color but an intense light. However, the association of whiteness/blackness in Nephite terms is a description of righteousness. (See commentary accompanying 2 Nephi 5:21.)