I'm not talking about the spells' color, Kuja. I'm talking about the fact that the spell description for Luminaire says "a dome of pure light." Not a dome of light and crackling electricity, not a dome of light and freezing magic, not a dome of light and cookies just light. That's all that Luminaire is. JUST LIGHT. It's a dome of magical light that for some reason causes lightning-based damage when it hits Magus. If a dome of nothing but light causes lightning-based damage to Magus, then a blade made of nothing but light will do the same. End of story.