Metal excess defect (Type 1): anion vacancy with trapped electron (F-centre). The electron absorbs visible light, giving colour. NaCl heated in Na vapour: yellow (F-centres). KCl in K vapour: violet. LiCl: pink. Metal excess defect (Type 2): extra cation in interstitial site + extra electron. Example: ZnO (turns yellow on heating, loses O). Metal deficiency defect: cation vacancy, compensated by neighbouring cation having higher oxidation state. Example: FeO (always non-stoichiometric, Fe0.95O). $Fe^{3+}$ replaces some $Fe^{2+}$ to compensate.