Q.The yellow colour in NaCl crystal is due to

NaCl heated in sodium metal vapour develops a metal-excess defect, where extra $Na^+$ ions occupy the lattice and the corresponding anion vacancies trap electrons (F-centres); AgBr, AgCl, and FeO are classic examples of different defect types.

A metal-excess defect (a type of non-stoichiometric defect) arises when a crystal has an excess of the metal ion relative to the ideal stoichiometric ratio. The textbook example is alkali halides such as NaCl: when NaCl crystals are heated in an atmosphere of sodium vapour, Na atoms deposit on the crystal surface and diffuse in; each deposits as $Na^+$ at a normal cation site, while the electron it releases occupies a nearby anion vacancy (creating what is called an F-centre, from German 'Farbzentrum'). These trapped electrons impart the characteristic yellow colour to excess-Na-heated NaCl. In contrast, AgCl and AgBr are the st …

In FeO, partial oxidation of $Fe^{2+}$ to $Fe^{3+}$ creates iron-ion vacancies to preserve overall charge neutrality, giving a non-stoichiometric, metal-deficient crystal ($Fe_{0.95}O$ typically) — a hallmark example of the metal deficiency defect.

If some $Fe^{2+}$ ions in the FeO lattice are oxidised to $Fe^{3+}$, the extra positive charge from each $Fe^{3+}$ means fewer total cations are needed to balance the fixed number of $O^{2-}$ ions, so some cation sites are left vacant — producing a crystal with fewer metal ions than the stoichiometric formula suggests, i.e. a metal-deficiency defect. NaCl and KCl are classic examples of the Schottky defect (equal numbers of cation and anion vacancies, stoichiometry unchanged), not meta …