This is blue because ion absorbs yellow radiation and transmits blue radiation unchanged. As we know that copper is a transition metal and we also know that all transition metals have coloured compounds due to their partially filled d orbitals. I hope you get it Please rate Solution This is blue because ion absorbs yellow radiation and transmits blue radiation unchanged. As we know that copper is a transition metal and we also know that all transition metals have coloured compounds due to their partially filled d orbitals. I hope you get it Please rate.