Copper chloride CuCl2 is typically green or blue-green in color. The specific color of CuCl2 is due to the electronic transitions in the copper ions Cu2+ . When light is absorbed by the Cu2+ ions, electrons are excited from lower energy levels to higher energy levels. The energy difference between these levels corresponds to the energy of the absorbed light, which determines the color we observe.In the case of CuCl2, the d-orbitals of the Cu2+ ions are partially filled, and the absorbed light corresponds to the energy difference between these orbitals. The remaining transmitted or reflected light appears green or blue-green to our eyes.