Why transition elements form a coloured compounds?

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 in two sets of orbitals normally i.e., $\mathrm{e}\left(d x^{2}-y^{2}, d z^{2}\right)$ and $\mathrm{t}_{2 g}(d x y, d y z, d x z)$ depending uponthe geometry of the complex.

These sets of orbitals have different energies after splitting and when an electron from low energy $d$-orbital is excited to higher energy $d$-orbital, the energy of excitation corresponds to the frequency of light absorbed.