Think of all the garments you own and imagine the things that they have to go through before you buy them and during use. Their colour has to resist fading when they are used or left out in sunlight. They have to be suitable for repeated washing without the colour running. But there are other factors as well.

For example, the colour in your swimwear must also be fast to sea water and the chlorinated water used in the swimming pool. The blouse and shirt you wear next to your skin should not discolour because of the effects of perspiration (which can be either acid or alkali).

These are the sort of issues that dye manufacturers need to consider before launching dyes onto the market.
Because of the wide range of end uses of coloured textiles, many tests have been developed to assess fastness. Testing involves comparing a dyed sample that has been exposed to an agency, for example to light or to washing, with an original, to assess accurately any change in shade or change in depth of colour. Up to a certain level, changes are considered by industry to be acceptable, depending on the end use of the dyed material. If these levels are exceeded the product fails the test.

In washing fastness a sample is tested with so-called adjacent fabrics to assess the extent of staining of a piece of white fabric, similar to what happens when coloured garments are washed together with whites in a washing machine.