The colour of KMnO4 is due to charge transfer transitions by absorption of visible light.
Why is KMnO4 coloured?
KMnO4 has a deep purple colour. The metal ions in KMnO4 contains d electron and, therefore, charge transfer occurs from O– to Mn+. In fact, the lowest energy L→M charge transfer is due to the transition of a nonbonding 2p oxygen electron to the unoccupied molecular orbital level of doing tetrahedral compound. As the positive charge on a metal ion increases, its electronegativity also increases. Therefore, a transfer of charge from O2- to Mn+ ion may occur in the lower energy region (visible region).