ChilliWilly, nice question. I'm sure there is a deep answer to be found through Google, but right off the top of my head, the first has to do with photo-chemistry and the second has to do with photo-biology.
The pigment in hair is broken down by high-energy light waves. In the upper atmosphere, gas molecules are photo-dissociated (split apart by those energetic photons). That's how we get ozone in the upper atmosphere; after that photo-dissociation, the lonely, but charged oxygen atoms decide to make a threesome. In hair, the chemical which acts as a pigment absorbs one set of frequencies. The reduction of that chemical by photo-dissociation means there is less of that chemical to absorb those frequencies. That's photo-chemistry.
The skin, when it detects ultraviolet, sends out melanin to protect it from those harsh rays. Melanin is a darker color. This is photo-biology.
That's my educated guess.