Why does table salt, some times appear yellow in colour?

Explanation

Heating NaCl in presence of sodium vapour results in non-stoichiometric sodium chloride `(Na_(1+x)Cl)` with metal excess defects arising from anion vacancies in the crystal. To maintain charge balance in crystal, vacant anionic sites are occupied by unpaired electrons. Anionic sites occupied by unpaired electrons are called F-centres. The electrons in the F-centres absorb energy from white light and undergo electronic transitions. Due to this transition, the colour of sodium chloride becomes yellow.