why is zinc oxide yellow when heated but white when cold?

ZnO is white when cold but turns yellow on heating. It returns to white when on cooling. The same situation for lead(II)oxide.
These divalent compounds have a complete d shell, so the colour is not from d-d spectra. Their colour is due to defects in the solid structure. On heating ZnO loses O. The number of defects increases with temperature and is zero at absolute zero.

Nonstoichiometric defects --- Metal excess
Interstital ions and electrons
Metal excess defects also occur when an extra positive ion occupies an interstitial position in the lattice, and electrical neutrality is maintained by the inclusion of an interstitial electron. This kind of metal excess defect is much more common in crystals where the ions are appreciably in size, have a low co-ordination number, and have some covalent character). Examples include ZnO, CdO, Fe2O3 and Cr2O3.

I hope this can help your understanding. =)

2007-09-20 10:03:13 補充：
The red and yellow forms of this material are related by a small change in enthalpy: PbO(red) → PbO(yellow) ΔH = 1.6 kJ/mol

2007-09-20 10:16:59 補充：
Lead(II) oxide, is the chemical compound with the formula PbO. Also known as, this compound exists in both tetragonal (litharge) and orthorhombic (massicot) forms, which are yellow and yellow to reddish-yellow earthy scaley masses, respectively.