When irradiated with UV light, the non stoichiometric ZnO undergoes following reaction:
ZnO----Zn^2+1/2O2+2e
The excess Zn^2 thus formed get trapped into the vacant interstitial sites and the electrons in the neighbouring interstitial sites; of course maintaing the electrical neutrality. These loosely elecrtons absorb different colors to emit Green, yellow, and orange colors. Such centres are called FARBE CENTRES- meaning color in German
REFERENCE:
A. B. Djurišić1,a), Y. H. Leung1, K. H. Tam1, L. Ding2, W. K. Ge2, H. Y. Chen3 and S. Gwo3