The colour of transition metals and their compound is due to which of the following?

presence of unpaired electrons

Usually the color of transition metal compound is due to d-d transition. When the d-level is not completely filled , it is possible to promote electron from a lower energy d-orbital to higher energy d-orbital by absorption of a photon of electromagnetic radiation having an appropriate energy.

Hence on the other hand we can say that the color of transition metal is due to incomplete (n-1) d- subshell.