Zinc oxide, white in color at room temperature, acquires yellow color on heating due to:

At room temperature, zinc oxide is white; but, when heated, it turns yellow as a result of reversible oxygen loss at very high temperatures. $Z{n}^{2+}+\frac{1}{2}{O}_2+2e^{-}$is created when zinc oxide is heated. zinc oxide in low-oxygen environments.

When heated, zinc oxide, which is colorless at ambient temperature due to the trapping of electrons at the site, turns yellow.