When ZnO is heated, it splits up to give Zn²⁺, electrons and colour because of the following reasons:

The excess Zn ions thus formed get entrapped in the interstitial site and electron in the neighborhood vacant interstitial sites. This electron is responsible for the colour and electrical conductivity in crystals.

ZnO → Zn²⁺ + 1/2O₂ + 2e^-

ZnO heated in Zn vapour → ZnyO (y > 1)

The excess cations occupy interstitial voids

The electrons (2e^-) released stay associated to the interstitial cation.