See the difference for yourself
You will receive 2 samples: Place one swatch in the sun, on a heater or use a hairdryer to warm up the swatch. Put the other sample in the fridge for a while and you will see when you put the samples side by side, that the cold sample is darker than the sample exposed to heat. You can keep testing this effect alternately
The discolouration can also be done with warm (not hot) and cold water.