What is the chemical process involved in human hairs changing colour after sun exposure?
Melanin is the main compound responsible for the color in hair. It is synthesized from the amino acid tyrosine. The absorption of tyrosine is around 280nm, which is a shorter wavelength than humans can see. Tyrosine can not absorb or preferentially scatter visible light in order to appear colored. It appears colorless as it scatters visible light equivalently. After tyrosine is synthesized into melanin, a pigment is formed which can be seen. Melanin scatters and absorbs light preferentially, it appears non-white.



The absorption of melanin is in the visible spectrum. This means it can absorb and scatter light in the visible spectrum differentially. 



As your hair is exposed to sunlight, melanin begins to break down into lower molecular weight compounds which have their own absorbance spectra. The different forms of melanin and carotenoids in your hair can be oxidized by sunlight, giving hair a spectrum of possible colors. 

Pigment in bleached hair has been oxidized into products absorbing light primarily outside the visible spectrum. White hair appears so because it lacks compounds which preferentially scatter and absorb light in the visible spectrum.