The tendency of the transition elements to form coloured compound is attributed to:

According to crystal field theory the colour of the complex is due to the d-d transition of the electron in presence of ligands. For example, in ${\left[\mathrm{Ti}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ by absorbing green yellow component of white light, d-d transition of electron takes place and the aqueous solution appears purple as the purple is the complimentary colour of greenish-yellow.