Heat sensitive plastic
This is a cheap simple exhibit and allows visitors to make a (tempory) mark on the exhibition. The plastic has a transition temperature and you may find that you need more than one version for summer and winter if the temperature of your gallery (and visitors) changes too much.
The film itself uses a form of liquid crystal, that creates a spiral, whose pitch changes with temperature. Depending on the pitch (and so the temperatures), different colours reflect due to bragg diffraction.