Why does the color of some clothes fade when dried in the sun
The process of applying color to clothes is called dyeing and this is effected by different means.The dyes used for applying color to clothes include acid dyes(having strong affinity for cellulose fiber), mordant dyes(which require addition of chemicals), azoic dyes, vat dyes etc. Depending on the chemical properties of the dyes, their mode of application is also different.

The bright colors on the clothes are as a result of absorption and reflection of sunlight by the molecules of the dye, which imparts color to the clothes. However over exposure of the dye molecules to sunlight can cause degradation commonly referred to as photochemical degradation. This results in the fading of colors on the clothes. The amount of photochemical degradation depends on the nature of the chemical used in the dye. It is due to this reason that some colors tend to fade faster than other colors.

Even though there are additives which claim to improve its resistance against photochemical degradation, their use is not very popular. Other methods that can be employed to prevent fading of colors on clothes include drying with clothes turned inside out, drying clothes in the shade, adding vinegar when washing, using cold water for washing etc.


Reference


[1] http://www.fibre2fashion.com/industry-article/17/1623/dyeing1.asp
[2] http://en.wikipedia.org/wiki/Photochemistry
[3] http://www.livingspace360.com/index.php/simple-tips-to-prevent-fading-of-clothes-remedies-for-long-        life-of-clothes-stop-color-fade-clothes-19400/
[4] http://www.ask.com/question/why-do-some-colors-fade-faster-than-others
[5] http://www.hindu.com/seta/2006/04/20/stories/2006042004241700.htm