The colours of the transition metal are due to d-d excitation. The energy required for d-d electron axcitation is available in the visible range.
Transition metal ions have the tendency to absorb certain rediations from the visible region and exhibit the complementary colour.
The transition metal ions which have completely filled d-orbitals are colourless as the excitation of electron or electrons is not possible within d-orbitals. The transition metal ions which have completely empthy d-orbitals are also colourless. In `KMnO_4` and `K_2Cr_2O_7`, there are no unpaired electrons at the central atom but they are dep in colour. The colour of these compounds is due to charge transfer spectrum. for example in `MnO_4` electron is momentrily transferred from O to the metal and thuys oxygen changes from `O^(2-)` and `O^(ɵ)` maganese from `Mn^(7+)` to `Mn^(6+)`.
Q. `K_2Cr_2O_7` gives coloured solution in water. The colour is due to