Colour is observed when NaCl is heated in the vapours of sodium.It is due to

Alkali halides like NaCl show metal excess defect. 

When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. 

The Cl^– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na⁺ ions. 

The released electrons diffuse into the crystal and occupy anionic sites . As a result the crystal now has an excess of sodium ion. The anionic sites occupied by unpaired electrons are called F-centres . They impart yellow colour to the crystals of NaCl.