Due to electronic transition, potassium permanganate is purple in colour. Its not due to d-d transition as Mn in KMnO4 exist with +7 oxidation state and it has no d electrons. Due to charge transfer reaction within the molecule, photons promote an electron from the highest energy MO orbital in one of the Mn-O bonds to an empty d orbital of Mn. This promotion is equivalent to the energy of a yellow photon, so yellow light is absorbed and make us to see as its complementary colour purple...