Which color has the lowest energy?

I'm not sure what you are talking about. Energy is not a physical unit. It is a philosophical concept and an accounting technique used to analyze mechanical and chemical exchanges. In nuclear physics it is defined as a color of light, in biology it is a synonym for metabolism, and in public utilities they say energy when they mean connectivity. In quantum physics, red would be less energy than other visible colors. But I doubt that you are talking about quantum physics.