The colour of the transition metal ions is due to

Explanation

The factors which affect colur of transition metal ions are:
(a) The nature of the ligand : Different ligands have different effects on the energies of the d orbital of the central ion. Some ligands have strong electrical fields which cause a large energy gap when a d orbitals split into two groups. Others have mush weaker fields producing much smaller gaps. The greater the splitting, the more energy is needed to promote an electron from the lower group of orbitals to the higher ones. In terms of the colour of the light absorbed, greater energy corresponds to shorter wavelengths. This means that as the splitting increases, the light absorbed will tend to shift away from the red end of the spectrum towards orange, yellow and so no.
(b) The oxidiation state of the metal : As the oxidation state of the metal increases, so does the amount of spliting of the d-orbitals. Changes of oxidation state therefore changes the colour of the light abosrtbed, and thus the colour of the light we see.
(c ) The coordination of the ion : Splitting is greater if the ion is octahedral than if it is tetrahedral , and therefore, the colour will change with the change of coordination.