Crystals of a substance changed their colour on heating in a closed test tube but regained it after some time when they were allowed to cool down. The substance is [[1]]

The crystals that change colour when heated in a closed tube but return to their original colour after some time do so because they scarcely have any impurities in them. The temperature might easily have an impact on them. These crystals often contain some water molecules and are produced from watery solutions.

Thus, the phenomenon of water of crystallisation is responsible for the mechanism by which crystals in a closed test tube change colour when heated then regain it when cooled. A set number of water molecules are contained in the substance in one salt formula unit, according to the phenomena of water crystallisation. The technique is also known as "water of hydration." As the material is hydrated by the water molecules.

On heating in a closed container the only possible reaction to take place is drying or water removal. Hydrates are compounds containing water molecules in their formula. These water molecule is removed on heating and the color is changed.

Hydrated copper sulfate,  ${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$ has a blue color. When water molecule is removed the color becomes white. The process involved here is water of crystallization.

${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}\stackrel{∆}{\to }{\mathrm{CuSO}}_4$

Hence, the answer is  Hydrated copper sulfate.