Color-changing materials have a variety of applications, ranging from smart windows to sensors. Here, we report deliquescent chromism of thin, color neutral films of nickel(II) iodide (NiI2) that are less than 10 mu m thick. This behavior does not occur in the bulk material. Dark brown thin films of crystalline NiI2 turn clear when exposed to humidity and can be switched back to the dark state when mildly heated (>35 degrees C). This optical transition between dark and clear states of an NiI2 thin film is reversible with thermal cycling.