$\therefore \ln M n O_{4}^{-},$ Mn has $+7$ oxidation state having no electron in d-orbitals. It ts considered that higher the oxidation state of metal, greater is the tendency to occur $L \rightarrow M$ charge transfer,
because ligand is able to donate the electron into the vacant ( - orbital of metal.
since, charge transfer is laporate as well as spin allowed, therefore, it shows colour