I think the idea about changing the color of one's hair via bionanotechnology is an interesting one. Certain cells are found in the hair follicles which release melanin. If one could somehow downregulate them and replace them with cells that are encoded to produce photoactive compounds that change color as a response to temperature/humidity, it would be pretty neat. Of course, as with all technologies, I expect the real driving forces would be military (can save on air conditioning in the Middle East) or smut (Scarlett Johansson goes naturally red in the summer adult blockbuster.....).

There are lots of issues about the types of melanin (there are three or four different types produced in humans) and differences in expression throughout the different tissue types (for example, there's a melanin produced in the brain and its presence/absence is observed in past studies of Parkinson's disease). I unfortunately don't know enough about tissue-specific biology to say anymore off the top of my head. The big issue is that whatever you would want to replace it with would have to mimic its photoprotective properties against UV radiation. Melanin absorbs UV radiation and converts it into heat (IR radiation), preventing massive amounts of free radicals from being produced, which could cause serious cellular and genetic damage. It wouldn't be enough for something to simply absorb UV radiation - it would have to dispose of it in a similar way.

Anyway, back to article drafting....