Answer: transition of an electron between d orbitals
Transition metals are defined as having partly filled d orbitals.
When an electron from a lower energy d orbital is excited to a higher energy d orbital, the energy of excitation corresponds to the frequency of light absorbed.
This frequency generally lies in the visible region. The colour observed corresponds to the complementary colour of the light absorbed.
The frequency of the light absorbed is determined by the nature of the ligand.