When crystals of KCl are heated in an atmosphere of Potassium vapours the colour imparted is _______ and it is due to __________ defect

In such cases, ${\mathrm{Cl}}^{-}$  ions diffuse to the surface;

To combine with these ${\mathrm{Cl}}^{-}$  ions the Potassium atoms of the vapour  lose electrons to form ${\mathrm{K}}^{+}$and thus  form KCl. 

The electrons lost by Potassium atoms occupy anionic sites.

This crystal now has anion vacancies. 

It is a metal excess defect. 

The anionic sites occupied by  unpaired electrons are called F-centres. They impart lilac colour to the crystal.