The colour fastness test is a key indicator of how long a dye or print will last and how it could change as a result of external influences. Depending on the area of use, textiles are dyed to be more or less colourfast. For instance, clothing must remain colourfast after being washed, while parasols must retain their colour despite being exposed to light. We can assess colour fastness using a "grey scale". This can reveal changes in the colour depth, tone or purity. Often, all these changes are present simultaneously. In this case, our testing is based on the extent of the visible contrast between a treated sample and an untreated sample.