Nanotech hair dye

Conventional hair dye consists of coloured particles, which lodge inside the hair shaft and reflect certain wavelengths of light. What if instead of dumb particles, one was to use nano-scale machines? Each one would have a rotor, with different sides reflecting different colours. An actuator, controlled by an onboard CPU or a simple receiver, would rotate the rotor.

Commands could be broadcast to all the 'bots through an extremely low-power RF transmitter, an analogue of Bluetooth, or an IP network implemented on the wearer's body's electrical field, from a pocket remote control unit.

In the simplest variant, the wearer could change their hair colour by remote control; you could have conservative brown hair in your day job at the bank and shift to fluorescent pink for a weekend of raving. Or if you are a salesperson, consultant, pick-up artist, conman, social worker or other people-oriented profession, you could tailor your appearance to best match your audience, more easily winning their approval or trust.

Of course, it wouldn't be long before someone pushed the envelope and animated the whole thing; one could, of course, repeatedly send colour-change commands from the controller, causing your hair to flash. Entire subcultural codes may emerge, using changes of hair colour for flirtation or some other form of social signalling.

For that matter, why not do away with the remote control (or switch it off) and make the nanobots into a cellular automaton of some sort, each bot's state governed by those in its immediate vicinity. Depending on the function used, one could have all sorts of colour-cycling flame effects.