Colour fastness refers to the ability of dyes or pigments on dyed articles to retain their original colour under various external influences during end use or post-dyeing processing. It is also a quality requirement for dyed and printed fabrics. During wear and storage, dyed fabrics may fade or discolour due to light, perspiration, rubbing, washing, ironing and other factors, impairing the aesthetic appearance of fabrics or garments. The nature and extent of colour change can be expressed by colour fastness. The colour fastness of fabrics is related to fibre type, yarn structure, fabric weave, printing and dyeing methods, dye variety and the magnitude of external forces. It can be classified into colour fastness to light, colour fastness to washing or soaping, colour fastness to rubbing, colour fastness to perspiration, colour fastness to ironing, colour fastness to sublimation, etc.

Colour fastness to light indicates the degree of discolouration of coloured fabrics exposed to sunlight. Testing can be carried out via natural sunlight exposure or artificial light fading equipment. The fading level of exposed specimens is compared against standard references. Two rating tools are available: blue wool standards and grey scale. Chinese national standards and European standards adopt an 8-grade system, where Grade 8 is the best and Grade 1 the poorest; American standards adopt a 5-grade system, with Grade 5 optimal and Grade 1 worst. Fabrics with poor light fastness must avoid prolonged direct sunlight and should be air-dried in ventilated shade. Xenon arc lamp exposure for 24 hours is commonly adopted for testing.

Colour fastness to washing or soaping describes the degree of colour change of dyed fabrics after washing with detergent solutions. A grey scale is normally used for evaluation, judging colour difference between the original sample and the washed specimen. Washing fastness is divided into 5 grades, Grade 5 being the best and Grade 1 the poorest. Fabrics with poor washing fastness are suitable for dry cleaning. If wet cleaning is required, strict control of washing conditions is necessary, such as avoiding excessive temperature and prolonged washing duration. Washing fastness covers two indicators: colour change of the original specimen and staining of adjacent white fabric. White fabric staining tests cover six fibre types: cotton, polyamide, polyester, wool, acrylic and acetate. Common test temperatures include 40°C, 50°C, 60°C and 95°C. The 40°C protocol applies generally to silk and wool, while 60°C is used for cellulosic fibres.

Colour fastness to rubbing denotes the colour transfer degree of dyed fabrics upon friction, divided into dry rubbing and wet rubbing. Evaluation is based on the staining level of white test cloth, graded from Grade 1 to Grade 5; higher grades represent better rubbing fastness.

Colour fastness to perspiration describes the fading extent of dyed fabrics immersed in perspiration. As artificially formulated perspiration solutions vary in composition, this property is often tested separately and also assessed in combination with other colour fastness indicators. Perspiration fastness is rated Grade 1 to 5, with higher grades indicating superior performance. Under Chinese national standards and European standards, it is further split into colour fastness to acid perspiration and colour fastness to alkaline perspiration.

Colour fastness to ironing refers to the degree of discolouration or fading of dyed fabrics during ironing. The colour change and fading are assessed by observing the staining transferred onto adjacent fabrics by the hot iron. It is graded from 1 to 5; Grade 5 is optimum and Grade 1 the lowest. Appropriate ironing temperatures shall be selected when testing different fabrics.

Colour fastness to sublimation represents the tendency of dyed fabrics to experience dye sublimation during storage. A grey scale is used to evaluate colour change, fading and white fabric staining after dry heat pressing, with 5 grades ranging from Grade 1 (worst) to Grade 5 (best). This test is mainly applied to polyester fabrics dyed with disperse dyes.

This test is mainly targeted at swimwear, as chlorine in seawater readily causes fabric fading. Tap water in cities also contains available chlorine. Articles in hospitals and hotels require disinfection with chlorine-containing detergents, leading to increasingly stringent requirements on colour fastness to chlorine bleach.

Some fabrics achieve satisfactory individual results for perspiration fastness and light fastness. However, under practical wearing conditions, especially in summer, fabrics easily fade under the combined effect of perspiration and sunlight. This triggered new performance requirements and corresponding testing standards.

Most modern laundry detergents contain peroxides such as peracetic acid together with activators, which cause textile fading via oxidation during washing. Therefore, textiles need to possess certain resistance to peroxides.

Ordinary fabrics generally require colour fastness of Grade 3–4 to meet daily wear requirements.

Additional fastness indicators include colour fastness to weathering, colour fastness to bleeding, colour fastness to saliva, colour fastness to water, colour fastness to dry cleaning and more. These indicators impose higher and higher technical demands on textile printing and dyeing processing.