Why do transition elements form coloured compounds?

(N/A) Most of the compounds of transition elements are coloured. This can be explained by $d-d$ transition.
In the presence of combining molecules or anions called ligands,the $d$-orbital loses its degeneracy and splits into two sets of orbitals,typically $e_g$ $(d_{x^2-y^2}, d_{z^2})$ and $t_{2g}$ $(d_{xy}, d_{yz}, d_{xz})$,depending upon the geometry of the complex.
These sets of orbitals have different energies after splitting. 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 from the visible region,resulting in the complementary colour being observed.