Transition metal complexes are typically coloured because of:

In a ligand field the originally degenerate d-orbitals split into sets ($t_{2g}$ and $e_g$ for octahedral). Promotion of an electron from a lower to a higher set absorbs photons in the visible range, and the colour we observe is the complementary colour of what is absorbed. The transition is forbidden by selection rules but slightly relaxed by vibronic and ligand-field effects, giving the characteristic pale-to-moderate colours.