How would you account for the following situations?
The transition metals generally form coloured compounds.

Transition element exhibit colour due to d-d transition, d-d transition is possible only when d- subshell have unpaired electron. Thus transition metal compounds are shows coloure because of the presence of one or more unpaired electrons in the d-orbital of the metal ion. A compound of transition metal appears coloured when one frequency of definite energy is absorbed (DE = hv) for d-d transition of the unpaired electron and other frequencies of visible region are transmitted. The colour of the compound corresponds to the frequency of the transmitted light. Thus, the observed colour (frequency) is complementary to the absorbed frequency.