Zinc oxide turns yellow on heating and regains its white colour on cooling. This is because

1. AAnions leave to form Schottky defects
2. BSome Zn${}^{2+}$ are oxidised to Zn${}^{3+}$ (metal deficiency)
3. COxygen enters interstitial sites as O${}^{2-}$
4. DZn${}^{2+}$ ions move to interstitial sites and free electrons are trapped there (metal excess)