Several d-metal complexes show colour. This is because their d-orbitals are not degerate in energy levels. Instead the d-orbitals split, some having higher energy than average and some lower than average. This split can lead to electrons in lower enegry d-orbitals to be promoted into higher enegry orbitals when they a struch with a phonton of light with a certain frequency. As light of a certain frequency is absorbed, the light of an opposite frequency is emitted by the complex. This emitted frequency is what gives the complex colour. However, this process can only occur is the complex has a patially filled d-orbotals, otherwise the promotion of electrons canot take place.