why does white Zn0(s) becomes yellow upon heating ?

Explanation

When ZnO is heated it loses oxygen as :
`ZnO overset("Heat")(rarr) Zn^(2+) + 1/2 O_2 + 2e^(-)`
The `Zn^(2+)` ions are entrapped in the neighbouring interstitial sites to maintained electrical neutrality. This results in metal excess defect and F-centres are created . Due to the presence of electrons in the interstitial voids, the colour is yellow.