On heating a crystal of kcl in potassium vapour, the crystal starts a violet colour.

why?

This happens due to creation of F-centers, as a result of Chloride anion vacancy created upon heating KCl in excess of K vapours. The hole created by the chloride anion is filled by an electron from K, giving it violet appearance.

In presence of excess of potassium, Cl- ions leave their lattice sites creating anion vacancies which are occupied by electrons given out by potassium.
These are called F- centres and are responsible for colour.of potassium,CI