On heating zinc oxide becomes yellow in colour. Why?

 On heaating it becomes yellow due to those electrons which jump to excited state....absorb colour opposite to yellow colour....and hence emmit yellow colour

  v  i

r    b

  o y  g  as seen in the circle of spectrum..........it may have a colour like  violet/indigo....and have emmited opposite colour.....ie.  YELLOW.

sry..i asked about which defect cause this condition..

this is metal excess defect

By the presence of extra cations at the interstitial sites

thank you amit....

At low temperature, the electronic configuration of Zn⁺² ion 3d¹⁰4s °. At elevated temperature, the transition of one electron takes from 3d to 4s by absorption of energy due to which ZnO appears yellow at elevated temperature.

 Metal defects due to presence of extra cations

In these type of defects, extra cations are present at the interstitial positions in the lattice. To maintain the electrical neutrality, extra electrons are present at the interstitial positions. These type of defects are shown by compounds such as ZnO, CdO, etc. When white zinc oxide is heated, it loses oxygen and turns yellow.