Why does ZnO turn yellow when it is hot?

ZnO is normally a white, finely divide material with the wurtzite structure. On heating, the color changes to yellow due to the evaporation of oxygen from the lattice to give a nonstoichiometric phase Zn1+xO (x </= 70 ppm); Indeed, by "doping " ZnO with an excess of 0.02-0.03% Zn metal, a whole range of colors - yellow, green, brown, red - can be obtained."