Question

why does table salt, NaCl sometimes appear yellow on color

Answer 1

The yellow colour of sodium chloride crystals is due to metal excess defect. In this defect, the unpaired electrons get trapped in anion vacancies. These sites are called F-centres. The yellow colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.

Answer 2

Explanation:

please find the above pic useful