Why does LiCl crystal containing excess lithium ions show pink colour?

Explanation

LiCl crystal with excess `Li^(+)` ions possesses metal excess defect which is associated with anion vacancies in the crystal. To maintain electrical neutrality of the crystal, vacant anion sites are occupied by unpaired electrons, forming what we call F-centres. The presence of F-centres in LiCl crystal with excess `Li^(+)` ions is responsible for imparting the pink colour.