Question

why the color of kcl turns white on heating

Answer 1

water of crystallisation leaves the KCl molecule, it's not just KCl but many other compounds too that lose their color on heating the water of crystallisation is what imparts Colors to many of these molecules.

Answer 2

When crystals of KCl are heated in an atmosphere of potassium vapour, the potassium atoms are deposited on the surface of the crystal. ... They impart violet colour to the crystals of KCl. Thecolour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.