Absorption/emission of light depends on the electronic configuration of an element.
When an atom absorbs a photon, one of the electrons goes from the ground state to an excited state. Then, it goes back to the ground state by emitting the photon it had absorbed, with the same wavelength (at least for simple cases). That wavelength depends on the energy gap between the ground state and the excited state.
The number of electrons for CuI
are different. So, the possibilities to excite an electron are different