Why is zinc oxide yellow when hot and white when cold?

The crystal structure changes with heating.

please explain elaborately.

Crystalline Zinc Oxide is thermochromic which means it has the ability to change colors based on temperatures. The color change has to do with reflection of certain wavelengths by the hexagonal crystals as it changes from a low temperature to a high temperature.

i like to know why a themochromic crystal changes its colour with temperature and how it is associated with reflection of wave length.

Here is an easy to understand article on Thermochromism

Thermochromic character of ZnO may be explained as follows. ZnO is a metal excess nonstoichiometric compound havinng a few excess Zn++ ions aswell as same no of eletrons in the interestitial positions of the crystal lattice.In heated condition the excess electrons in the interestitial positions of the crystal lattice get excited by absorbing selected wave lengths of the incident beam of white light emitting the complementary part and crystal appears yellow in our eye. In cold condition this does not occur and we see the crystal white. Experts comments in this matter are welcome